The correct answer is D.
Demand is the willingness and ability of consumers to purchase a good or service at a given price. It is a key concept in economics, and it is used to explain how markets work.
The willingness to purchase refers to the fact that consumers must be willing to buy a good or service in order for there to be demand. This willingness is based on a number of factors, including the price of the good or service, the consumer’s income, and the consumer’s preferences.
The ability to purchase refers to the fact that consumers must have the financial resources to buy a good or service. This ability is based on the consumer’s income, assets, and debts.
When both the willingness and ability to purchase are present, there is said to be a demand for a good or service. The amount of demand is determined by the number of consumers who are willing and able to purchase the good or service at a given price.
The other options are incorrect because they do not fully capture the concept of demand. Option A, willingness to purchase, is only one part of demand. Option B, consent to purchase, is not necessary for demand to exist. Option C, ability to pay, is also only one part of demand.