The relation between the consumer's optimal choice of quantity of a good and its price is denoted by :

Demand function

The correct answer is option (2) : Demand function

If the prices of other goods, the consumer’s income and her tastes and preferences remain unchanged, the amount of a good that the consumer optimally chooses, becomes entirely dependent on its price. The relation between the consumer’s optimal choice of the quantity of a good and its price is very important and this relation is called the demand function. Thus, the consumer’s demand function for a good gives the amount of the good that the consumer chooses at different levels of its price when the other things remain unchanged.

The consumer’s demand for a good as a function of its price can be written as X = f (P).