Demand is defined as
answer
the number of consumers in a market.
the quantity of products that consumers are willing to buy at different prices.
the amount of products produced.
the annual quantity of products sold by retailers.

Demand is the quantity of products that the consumers want or need and are willing to buy: the number of books, or the amount of fruit juice for example.

the correct answer is this one:

the quantity of products that consumers are willing to buy at different prices.