The correct answer is A.
A demand curve is a graph that shows the relationship between the price of a good and the quantity demanded of that good. When the demand curve shifts to the right, it means that consumers are willing to buy more of the good at any given price. This can be caused by a number of factors, such as a decrease in the price of the good, an increase in the income of consumers, or an increase in the number of consumers.
Option B is incorrect because the demand function is a mathematical equation that shows the relationship between the price of a good and the quantity demanded of that good. The demand function does not shift when the price of the good changes.
Option C is incorrect because the supply function is a mathematical equation that shows the relationship between the price of a good and the quantity supplied of that good. The supply function does not shift when the price of the good changes.
Option D is incorrect because the real income of the consumer is the amount of money that the consumer has to spend after adjusting for inflation. The real income of the consumer does not affect the demand curve.