The marginal seller is the seller:
a. for whom the marginal cost of producing one more unit of output is the lowest among all sellers, and the marginal buyer is the buyer whom the marginal benefit of one more unit of the good is the highest among all buyers
b. who supplies the smallest quantity of the good among all sellers, and the marginal buyer is the buyer who demands the smallest quantity of the good among all buyers
c. who would leave the market first if the price were any lower, and the marginal buyer is the buyer who would leave the emarket first if the price were any higher.

b. who supplies the smallest quantity of the good among all sellers, and the marginal buyer is the buyer who demands the smallest quantity of the good among all buyers

Explanation:

Marginal sellers and buyers are one who sells at a price that is lower than the other and barley sells in the market. Thus he sells his goods at the economic costs and does not earn a surplus.

Thus he has to maintain a margin within the market he can also leave the market if the prices tend to be lower.