Which one of the following may lead to a movement along the demand curve of a commodity?

1. The Law of Demand and Demand Schedule (basic)

Welcome to your first step in mastering Microeconomics! To understand how markets function, we must first look at the behavior of the consumer. The Law of Demand is a fundamental principle which states that, other things being equal (ceteris paribus), there is a negative or inverse relationship between the price of a commodity and the quantity demanded of it. In simpler terms, when the price of a good rises, the quantity demanded falls; conversely, when the price drops, the quantity demanded increases Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.24.

Why do we behave this way? Economists point to two primary reasons. First is the Law of Diminishing Marginal Utility. As you consume more units of a good, the satisfaction (utility) you derive from each additional unit decreases. Therefore, you would only be willing to buy that extra unit if its price is lower Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.11. Second is the Income Effect: when the price of a good falls, your purchasing power increases—meaning you feel "richer" with the same amount of money—allowing you to buy more of that good Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.24.

To visualize this, we use a Demand Schedule—a table showing the quantities of a good that a consumer is willing to buy at various prices. When we plot this data on a graph (with Price on the vertical axis and Quantity on the horizontal axis), we get a Demand Curve. Because of the inverse relationship between price and quantity, this curve always slopes downwards from left to right Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.10.

Price (₹ per unit)	Quantity Demanded (Units)	Observation
₹50	10	High price, low quantity
₹40	15	Price falls, quantity rises
₹30	25	Further fall, further rise

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.24; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.11; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.10

2. Determinants of Demand (basic)

When we talk about Demand, we aren't just looking at how much people want a product; we are looking at the quantity they are willing and able to buy at specific prices, given their income and preferences Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 10. While price is the most visible factor, demand is actually influenced by a variety of "determinants." Understanding these is crucial because economists distinguish between a change in the product's own price and changes in other external factors.

The primary determinant is the Price of the Commodity. If only the price of the good changes (while everything else remains constant), we see a movement along the demand curve. A fall in price leads to an "expansion" (downward movement), and a rise leads to a "contraction" (upward movement). However, when factors other than price change, the entire demand curve shifts because the consumer’s behavior has fundamentally changed at every possible price point Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 25-26.

Key determinants that cause these shifts include:

• Consumer Income: For Normal Goods, demand increases as income rises. However, for Inferior Goods (like certain coarse cereals), demand actually falls as consumers get richer and switch to better alternatives Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 24-25.
• Prices of Related Goods: These come in two flavors. Substitutes (like tea and coffee) have a direct relationship—if coffee gets expensive, tea demand shifts right. Complements (like tea and sugar) have an inverse relationship—if sugar prices skyrocket, tea demand might shift left Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 25.
• Tastes and Preferences: Changes in fashion, health awareness, or seasonal trends can shift demand regardless of price Microeconomics (NCERT class XII 2025 ed.), Chapter 5: Market Equilibrium, p. 76.

Factor Change	Impact on Curve	Technical Term
Price of the good itself falls	Downward movement along curve	Expansion of Demand
Consumer Income rises (Normal Good)	Entire curve shifts Right	Increase in Demand
Price of a Substitute rises	Entire curve shifts Right	Increase in Demand

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.10, 24, 25, 26; Microeconomics (NCERT class XII 2025 ed.), Chapter 5: Market Equilibrium, p.76

3. Classification of Goods: Normal, Inferior, and Giffen (intermediate)

In our study of demand, we must understand that not all goods react to changes in a consumer's circumstances in the same way. The most fundamental classification depends on how a consumer’s demand changes when their income changes. For Normal Goods, there is a direct relationship: as a consumer's income increases, their demand for the good also increases, shifting the demand curve to the right Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.24. Most items we consume daily, like branded clothes or fresh fruits, fall into this category because higher purchasing power allows us to afford more of them.

However, there are exceptions called Inferior Goods. For these goods, demand moves in the opposite direction of income. If a consumer becomes wealthier, they often reduce their consumption of low-quality items, such as coarse cereals (like bajra or maize), to switch to superior alternatives like wheat or rice Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.25. Interestingly, a good isn't inherently 'inferior' by its nature; it depends on the consumer's income level. A low-quality grain might be a normal good for someone at a very low income level (as they can finally afford to eat more of it), but it becomes an inferior good once their income rises enough to afford better food.

The most extreme case is the Giffen Good, a special type of inferior good that defies the basic Law of Demand. For a Giffen good, when the price increases, the demand also increases. This happens because the negative income effect of the price rise (which makes the consumer feel much poorer) is so strong that it outweighs the substitution effect (the tendency to buy cheaper alternatives). The consumer is forced to spend more on the basic Giffen good because they can no longer afford any luxury items at all. While all Giffen goods are inferior goods, not all inferior goods are Giffen goods.

Type of Good	Income Increase	Price Increase
Normal Good	Demand Increases	Demand Decreases (Law of Demand)
Inferior Good	Demand Decreases	Demand Decreases (Usually)
Giffen Good	Demand Decreases	Demand Increases (Paradox)

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.24; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.25

4. Related Goods: Substitutes and Complements (intermediate)

In our journey through demand theory, we've seen how a good's own price affects its quantity demanded. However, in the real world, goods don't exist in isolation. The demand for a product is often deeply influenced by the prices of related goods. Economists categorize these relationships into two primary types: Substitutes and Complements. Understanding these is vital because they determine how the entire demand curve shifts in response to external market changes Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 25.

Substitutes are goods that can be used in place of one another to satisfy the same want. Think of tea and coffee, or Pepsi and Coca-Cola. Because they are rivals for the consumer's budget, there is a positive relationship between the price of one and the demand for the other. If the price of coffee rises, consumers will find tea relatively cheaper and switch their consumption toward it. Consequently, the demand for tea increases, shifting its demand curve to the right, even though the price of tea itself hasn't changed Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 35.

Complements, on the other hand, are goods that are consumed together to provide utility—like a printer and ink cartridges, or a car and fuel. Here, the relationship is inverse. If the price of ink cartridges skyrockets, people will likely buy fewer printers because the total cost of "printing" has gone up. Therefore, an increase in the price of a complementary good causes the demand curve for the primary good to shift to the left Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p. 25.

Feature	Substitutes	Complements
Nature of Goods	Used instead of each other.	Used together with each other.
Price-Demand Link	Positive (Price of Y ↑ → Demand for X ↑)	Inverse (Price of Y ↑ → Demand for X ↓)
Examples	Tea & Coffee; Pens & Pencils	Cars & Petrol; Bread & Butter

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.25; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.35

5. Price Elasticity of Demand (intermediate)

Price Elasticity of Demand (eD) is a measure that quantifies the responsiveness of the quantity demanded of a good to a change in its price. While the Law of Demand tells us that demand moves in the opposite direction of price, elasticity tells us by how much it moves. It is formally defined as the percentage change in the demand for the good divided by the percentage change in its price Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.28. If the percentage change in demand is greater than the percentage change in price (eD > 1), the demand is elastic; if it is less (eD < 1), it is inelastic.

What determines this responsiveness? The most critical factor is the availability of close substitutes. For instance, while the general demand for food is inelastic (you must eat to survive), the demand for a specific variety of pulses is likely to be elastic because if its price rises, you can easily switch to a different variety Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.31. Similarly, luxury goods usually show high elasticity because consumers can postpone or forgo their purchase when prices rise, whereas necessities show low elasticity.

Type of Elasticity	Condition	Typical Example
Elastic	eD > 1	Luxury cars, specific brands of electronics
Inelastic	eD < 1	Life-saving medicines, salt, basic food staples
Unitary Elastic	eD = 1	Where % change in price = % change in quantity

It is a common misconception that elasticity is the same as the slope of the demand curve. On a linear (straight-line) demand curve, the elasticity actually varies at every single point. We can measure this geometrically: the elasticity at any point on the line is the ratio of the lower segment to the upper segment of the demand curve Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.30. This means demand is highly elastic at higher prices (top of the curve) and becomes increasingly inelastic as we move down toward lower prices.

Sources: Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.28; Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.29; Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.30; Microeconomics (NCERT class XII 2025 ed.), Theory of Consumer Behaviour, p.31

6. Movement vs Shift in the Demand Curve (exam-level)

In our journey through demand theory, it is crucial to distinguish between a change in the quantity people want because of a price change, and a change in demand because the world around the consumer has changed. We call this the distinction between a movement along the curve and a shift of the curve. This is not just semantics; it is the foundation of how economists analyze market changes.

A movement occurs exclusively when the price of the commodity itself changes, while all other factors (like income or tastes) remain constant. When the price of a good falls, consumers naturally buy more, leading to a downward movement along the curve known as an expansion or extension of demand. If the price rises, consumers buy less, causing an upward movement called a contraction. As noted in Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.27, the demand for a good generally moves in the opposite direction of its price, and this responsiveness is what we track along the existing curve.

A shift, however, happens when the underlying conditions of the market change. Factors such as a rise in consumer income, a change in fashion, or a change in the price of a substitute good (like tea vs. coffee) mean that consumers will now want to buy different quantities at every possible price level. This causes the entire demand curve to relocate. A rightward shift signifies an increase in demand, while a leftward shift signifies a decrease in demand Microeconomics (NCERT class XII 2025 ed.), Chapter 2, p.25. To keep your analysis sharp, always ask: "Is the price of the good itself changing, or is it something else?"

Feature	Movement along Demand Curve	Shift of Demand Curve
Primary Cause	Change in the price of the good itself.	Change in factors other than price (Income, Tastes, etc.).
Economic Term	Change in Quantity Demanded.	Change in Demand.
Visual Result	Sliding along the same line (Expansion/Contraction).	Drawing a completely new line (Right/Left shift).

Sources: Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.25; Microeconomics (NCERT class XII 2025 ed.), Chapter 2: Theory of Consumer Behaviour, p.27

7. Solving the Original PYQ (exam-level)

Now that you have mastered the fundamental principles of consumer behavior, this question tests your ability to distinguish between movements and shifts. The core building block here is the relationship between the variables plotted on the axes of your graph. As you learned in Microeconomics (NCERT class XII 2025 ed.), a demand curve is a graphical representation of the inverse relationship between the price of a commodity and the quantity a consumer is willing to buy, assuming all other factors remain constant (ceteris paribus). Therefore, if the only thing changing is the price of the good itself, we are simply sliding from one point to another on the existing line.

To arrive at the correct answer, (A) Change in its price, you must visualize the graph: when the price falls, we move downward along the curve (expansion), and when the price rises, we move upward (contraction). This is strictly defined as a change in quantity demanded. Your reasoning should be: if the variable causing the change is already on the Y-axis (Price), it cannot push the curve to a new location; it can only change our position on the current one. This is a vital distinction in economic theory that separates a simple price reaction from a structural change in market behavior.

UPSC frequently uses options (B), (C), and (D) as traps because they do influence how much of a product is bought, but they do so by shifting the entire curve. Changes in the price of related commodities, consumer income, or tastes and preferences are external factors that redefine the consumer's entire demand profile. When these change, the consumer will demand a different quantity at every possible price level, necessitating a brand-new curve. Remember: a change in the price of the good itself causes a movement, while a change in any other determinant causes a shift.