The correct answer is option 1: price of a commodity falls. Normally, the demand curve does not shift when the price of a commodity falls.

Option 1: When the price of a commodity falls, it leads to a movement along the demand curve rather than a shift. This is known as a change in quantity demanded. As the price decreases, consumers are willing and able to buy more of the commodity.

Option 2: If consumers want to buy more of a commodity at any given price, it would cause a shift in the demand curve. This indicates an increase in demand because consumers are willing and able to buy more at each price level.

Option 3: When average income rises, it can lead to a shift in the demand curve. An increase in average income usually results in an increase in purchasing power, leading to a higher demand for goods and services.

Option 4: A growing population can also cause a shift in the demand curve. A larger population means a larger pool of potential consumers, which can increase the demand for goods and services.

In summary, while options 2, 3, and 4 can cause a shift in the demand curve, option 1 (price of a commodity falls) does not lead to a shift but rather a movement