Concept: Demand is the quantity of a commodity that a consumer is willing and able to buy at a particular time and price.Explanation: The key phrase in the question is "willing and able". "Willing" means the consumer has the desire for the commodity. "Able" means the consumer has the purchasing power to pay for it. A mere wish or desire is not enough to count as demand. Only when both willingness and ability are present is the term demand used. Therefore, the correct option is that the commodity quantity is called demand.Answer: C. demand