The correct answer is: A. The desire, ability, and willingness of an individual to purchase goods and services at any given price per unit.
The demand for a good or service is the amount of that good or service that consumers are willing and able to purchase at various prices during a given period of time. The law of demand states that, all other things being equal, the quantity demanded of a good or service will decrease as the price of that good or service increases.
The three elements of demand are quantity, price, and time. The quantity demanded is the amount of a good or service that consumers are willing and able to purchase at a given price. The price is the amount of money that consumers must pay for a good or service. The time period refers to the length of time over which the demand is being considered.
Option B is incorrect because it does not include the element of price. Option C is incorrect because it does not include the element of time. Option D is incorrect because it is not one of the elements of demand.