Explanation:

When MU is zero, TU is at its maximum.

Understanding Total Utility (TU):
- Total Utility refers to the total satisfaction or happiness derived from consuming a certain quantity of a good or service.
- It is the sum of the marginal utilities of all units consumed.

Understanding Marginal Utility (MU):
- Marginal Utility refers to the additional satisfaction or happiness derived from consuming one additional unit of a good or service.
- It represents the change in total utility when one more unit is consumed.

Explanation of the Correct Answer:
- When MU is zero, it means that the additional satisfaction or happiness derived from consuming one more unit of the good or service is zero.
- This implies that consuming one more unit does not increase the total satisfaction or happiness.
- Therefore, the total utility is at its maximum at this point because any further increase in consumption would not lead to any additional satisfaction.
- In other words, the maximum total utility is achieved when the marginal utility of the last unit consumed is zero.

Example:
- Let's consider the consumption of chocolate bars. Initially, with each additional chocolate bar consumed, the consumer experiences an increase in satisfaction (marginal utility) and the total utility also increases.
- However, as the consumer consumes more and more chocolate bars, the marginal utility starts to diminish. Eventually, the consumer reaches a point where the marginal utility of consuming an additional chocolate bar becomes zero.
- At this point, the total utility is at its maximum because any further consumption of chocolate bars would not increase the total satisfaction.

Conclusion:
- When MU is zero, it indicates that the additional satisfaction derived from consuming one more unit is zero.
- Therefore, the total utility is at its maximum because any further increase in consumption would not lead to any additional satisfaction.