The correct answer is: A. Zero.
Marginal utility is the additional satisfaction or benefit that a consumer gains from consuming one more unit of a good or service. Average utility is the total utility divided by the number of units consumed.
When total utility is maximum, marginal utility is zero. This is because the consumer is no longer gaining any additional satisfaction from consuming more of the good or service. In other words, the consumer is at a point of satiation.
Option B is incorrect because marginal utility can be less than average utility even when total utility is maximum. This is because the consumer may be consuming more units of the good or service than they need to reach a point of satiation.
Option C is incorrect because marginal utility can be equal to average utility even when total utility is not maximum. This is because the consumer may be consuming the optimal amount of the good or service.
Option D is incorrect because marginal utility can be above average utility even when total utility is not maximum. This is because the consumer may be consuming less units of the good or service than they need to reach a point of satiation.