The correct answer is: A. Flow concept
Quantity demanded is a flow concept because it refers to the amount of a good or service that consumers are willing and able to purchase per unit of time. For example, if a consumer is willing and able to purchase 10 apples per week, then the quantity demanded of apples is 10 apples per week.
A stock concept, on the other hand, refers to the total amount of a good or service that is available at a given point in time. For example, if a store has 100 apples in stock, then the stock of apples is 100 apples.
It is important to note that quantity demanded and stock are related, but they are not the same thing. Quantity demanded is a flow concept that refers to the amount of a good or service that consumers are willing and able to purchase per unit of time. Stock is a stock concept that refers to the total amount of a good or service that is available at a given point in time.