The correct answer is: C. zero.
Marginal utility is the additional satisfaction or benefit that a consumer gains from consuming one more unit of a good or service. As a consumer consumes more and more units of a good, the marginal utility of that good decreases. This is because the consumer becomes less and less satisfied with each additional unit of the good.
At the maximum level of satisfaction, the marginal utility of the good is zero. This means that the consumer is no longer gaining any additional satisfaction from consuming more of the good.
Option A is incorrect because the marginal utility of a good cannot be positive at the maximum level of satisfaction. This is because the consumer is no longer gaining any additional satisfaction from consuming more of the good.
Option B is incorrect because the marginal utility of a good cannot be negative at the maximum level of satisfaction. This is because the consumer would not continue to consume the good if it was causing them dissatisfaction.
Option D is incorrect because the marginal utility of a good cannot be infinity at the maximum level of satisfaction. This is because the consumer would not be able to consume an infinite number of units of the good.