The correct answer is: b) A sale on shoes leads to higher purchases of shoes.
This scenario exemplifies a change in the quantity demanded because it demonstrates how a change in price—specifically a sale—affects consumer behavior. When shoes go on sale, their price decreases, prompting consumers to buy more pairs than they would at a higher price, illustrating the law of demand.
On the other hand, option a, which mentions an increase in salary leading to increased spending on clothing, is an example of a change in demand, not quantity demanded, as it indicates a broader shift in purchasing power and preferences rather than a change in the quantity demanded due to price. Similarly, option c, that a rise in the price of peanut butter leads to higher demand, also reflects a change in demand rather than a change in quantity demanded, as it describes the expected behavior of consumers in response to a price increase, typically reducing quantity demanded instead.