Suppose the market demand curve for a good passes through the point (quantity demanded = 100, price = $25). If there are five buyers in the market, then a. the marginal buyer's willingness to pay for the 100th unit of the good is $25. b. the sum of the five buyers' willingness to pay for the 100th unit of the good is $25. c. the average of the five buyers' willingness to pay for the 100th unit of the good is $25. d. all of the five buyers are willing to pay at least $25 for the 100th unit of the good.

Microeconomics basically works on the margin, it studies marginal costs, marginal revenue, marginal prices, marginal demand, marginal supply, etc. The margin measures the effect of one additional unit: either sold, consumed, produced, etc.

In this case, the marginal price of the 100th unit of the good is $25, that means that a buyer (you can call him a marginal buyer) will be willing and able to pay $25 for that specific unit of the good.

That doesn't mean that the price of the good is constant, both the supply and demand of goods are curves, because the marginal demand constantly changes depending on the marginal price and the marginal utility produced by consuming the extra unit of the good. On the other hand, the marginal changes depending on the marginal costs of producing that good, and the marginal revenue expected to be earned by selling that additional unit.