The correct answer is option-2 "negative." The slope of a normal demand curve represents the rate of change of quantity demanded with respect to a change in price. A negative slope indicates an inverse relationship between price and quantity demanded. This means that as the price of a good increases, the quantity demanded decreases, and vice versa.

Option-1 "positive" is incorrect because it implies a direct relationship between price and quantity demanded, which is not the case for a normal demand curve.

Option-3 "zero" is incorrect because it would indicate that there is no change in quantity demanded even when the price changes, which is not realistic for most goods.

Option-4 "infinity" is incorrect as it does not make sense in the context of a demand curve. Infinity implies an infinite rate of change, which is not applicable to the relationship between price and quantity demanded.

Therefore, the correct answer is option-2 "negative," indicating the inverse relationship between price and quantity demanded on a normal demand curve.