Explanation:

- Option:1, Unity: Marginal utility is not at unity when total utility reaches its peak. Unity refers to when marginal utility equals one, which usually applies in a different context.

- Option:2, Negative: When marginal utility is negative, it means additional consumption decreases total utility. This occurs when past the point of maximum total utility.

- Option:3, Zero: At the point where total utility is maximized, consuming additional units does not increase utility further. Hence, the marginal utility becomes zero. This is the correct answer.

- Option:4, Positive: Before reaching maximum total utility, adding more consumption would still increase total utility, keeping marginal utility positive.