The correct answer is: B. Additional.
Marginal is an adjective that means “of, relating to, or forming a margin or edge.” In economics, marginal refers to the additional benefit or cost that arises from a change in an activity. For example, the marginal cost of producing one more unit of a good is the additional cost that is incurred in producing that unit.
The other options are incorrect. Option A, “unimportant,” is incorrect because marginal is often used to refer to something that is very important. For example, the marginal cost of producing a good is often a key factor in determining whether or not to produce that good. Option C, “the minimum unit,” is incorrect because marginal can refer to any additional amount, not just the minimum amount. Option D, “just barely passing,” is incorrect because marginal does not have this meaning in economics.