The correct answer is D. Zero.
Marginal utility is the additional satisfaction or benefit that a consumer receives from consuming one more unit of a good or service. At the point of satiety, the consumer has consumed so much of the good or service that they no longer receive any additional satisfaction from consuming more. In other words, the marginal utility of the good or service is zero.
Option A is incorrect because marginal utility cannot be negative. This is because consumers will always choose to consume goods and services that provide them with positive utility.
Option B is incorrect because marginal utility can only be positive or zero. It cannot be negative.
Option C is incorrect because marginal utility is not always at its maximum at the point of satiety. In fact, marginal utility is often at its maximum before the point of satiety.
I hope this explanation is helpful. Please let me know if you have any other questions.