The correct answer is A. 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 B is incorrect because marginal utility can never be negative. This is because consumers will always choose to consume goods and services that provide them with positive utility. If the marginal utility of a good or service were negative, the consumer would be better off not consuming any of the good or service.
Option C is incorrect because marginal utility can never be maximum. This is because the marginal utility of a good or service will always decline as the consumer consumes more of the good or service. This is because the consumer will become less and less satisfied with each additional unit of the good or service.
Option D is incorrect because marginal utility can never be undefined. This is because marginal utility is a measure of satisfaction, and satisfaction is always a well-defined concept.