The correct answer is B. decreases.
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.
When marginal utility is negative, it means that the consumer is receiving less satisfaction from consuming an additional unit of the good or service. This can happen when the consumer has already consumed a lot of the good or service and is starting to feel full or when the good or service is no longer providing the consumer with any satisfaction.
When marginal utility is negative, total utility will decrease. This is because the consumer is receiving less satisfaction from consuming an additional unit of the good or service, so the total amount of satisfaction that they are receiving from consuming all units of the good or service will decrease.
Option A is incorrect because total utility can only increase if marginal utility is positive. Option C is incorrect because total utility can never be zero. Option D is incorrect because total utility can only be negative if marginal utility is negative.