The correct answer is A. When the total utility is maximum, marginal utility is 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.
The law of diminishing marginal utility states that as a consumer consumes more and more units of a good or service, the marginal utility of each additional unit decreases. This is because the consumer becomes less and less satisfied with each additional unit of the good or service.
When the total utility is maximum, the marginal utility is zero. This is because the consumer is no longer receiving any additional satisfaction from consuming more of the good or service.
Option B is incorrect because when total utility decreases, marginal utility can be positive, negative, or zero.
Option C is incorrect because total utility and marginal utility are related. Marginal utility is the change in total utility that results from consuming one more unit of a good or service.
Option D is incorrect because only option A is true.