Option 1: Supply function -> This relates to producers and shows the relationship between price and quantity supplied.
Option 2: Demand Function -> This shows the relationship between the price of a good and the consumer's optimal quantity choice.
Option 3: Cost Function -> This relates to production costs, not consumer choice.
Option 4: Output function -> This relates to production output, not consumer demand.
Hence, Option 2: Demand Function -> The demand function represents the relationship between a consumer's optimal choice of quantity and the price of a good. It is derived from utility maximization, where consumers choose quantities that maximize their satisfaction subject to their budget constraint. As price changes, the consumer adjusts their optimal quantity, creating the demand curve. This is a fundamental concept in consumer theory. -> correct