The correct answer is: A. MU is negative.
Marginal utility (MU) is the additional satisfaction or benefit that a consumer receives from consuming one more unit of a good or service. Total utility (TU) is the total satisfaction or benefit that a consumer receives from consuming all units of a good or service.
The MU curve will intersect the x-axis when MU is negative. This is because when MU is negative, the consumer is receiving less satisfaction from consuming an additional unit of the good or service. As a result, the consumer will not continue to consume the good or service, and the MU curve will intersect the x-axis.
Option B is incorrect because TU is maximum when MU is equal to zero. This is because when MU is equal to zero, the consumer is receiving the maximum amount of satisfaction from consuming the good or service. As a result, the consumer will not continue to consume the good or service, and the TU curve will reach a maximum point.
Option C is incorrect because both A and B are not always true. For example, if the consumer’s MU is negative, but their TU is still increasing, then the MU curve will not intersect the x-axis.
Option D is incorrect because the MU curve will intersect the x-axis when MU is negative.