At the highest level of total utility, marginal utility is

At the highest level of total utility, the marginal utility is zero. This is because total utility is the sum of marginal utilities, and when marginal utility reaches zero, it means that the consumer has reached the highest level of satisfaction they can derive from consuming that particular good or service. After this point, any additional units consumed will not increase the total utility, hence the marginal utility is zero. Therefore, the answer is option D: zero.