The correct answer is: C. Zero.
Marginal utility is the additional satisfaction or benefit that a consumer receives from consuming one more unit of a good or service. Total utility is the total satisfaction or benefit that a consumer receives from consuming all units of a good or service.
According to the law of diminishing marginal utility, as a consumer consumes more and more units of a good or service, the marginal utility of each additional unit decreases. This means that the consumer receives less and less satisfaction from each additional unit consumed.
When total utility is maximized, marginal utility is zero. This is because the consumer is receiving the maximum amount of satisfaction from consuming the good or service, and any additional consumption would result in a decrease in satisfaction.
Option A is incorrect because minimum marginal utility is not necessarily equal to zero. Option B is incorrect because average utility is not necessarily equal to zero. Option D is incorrect because negative marginal utility is not possible.