The correct answer is A. Marshall.
Alfred Marshall was an English economist who is considered one of the most influential economists of the 19th century. He is best known for his work on microeconomics, including his development of the concept of consumer surplus.
Consumer surplus is the difference between the maximum amount that a consumer is willing and able to pay for a good and the price that they actually pay. It is a measure of the benefit that consumers receive from consuming a good.
Marshall developed the concept of consumer surplus in his book “Principles of Economics,” which was published in 1890. He argued that consumer surplus is an important concept because it helps to explain how markets work.
In a competitive market, the price of a good will be equal to the marginal cost of production. The marginal cost of production is the cost of producing one additional unit of a good.
If the price of a good is equal to the marginal cost of production, then consumers will only be willing to buy the good if the benefit that they receive from consuming it is greater than or equal to the price.
However, if the price of a good is below the marginal cost of production, then consumers will be willing to buy the good even if the benefit that they receive from consuming it is less than the price.
This is because consumers will receive a surplus of benefit from consuming the good. The consumer surplus is the difference between the maximum amount that a consumer is willing and able to pay for a good and the price that they actually pay.
Consumer surplus is an important concept because it helps to explain how markets work. It also helps to explain why consumers are willing to pay a price for a good that is above the marginal cost of production.
The other options are incorrect because they are not the economists who developed the concept of consumer surplus.