In this question, we are asked to identify the incorrect statement among the given options regarding total utility and marginal utility.

Option 1 states that when total utility is maximum, marginal utility is zero. This statement is correct. When total utility is at its maximum, it means that the consumer is deriving the maximum satisfaction from the given quantity of a good. At this point, the marginal utility derived from consuming an additional unit of the good becomes zero because all the possible satisfaction has already been attained.

Option 2 states that when total utility is decreasing, marginal utility is negative. This statement is also correct. When total utility is decreasing, it means that the consumer is deriving less satisfaction from consuming additional units of the good. In this case, the marginal utility becomes negative because the additional units are reducing the overall satisfaction.

Option 3 states that when total utility is increasing, marginal utility is positive. This statement is correct. When total utility is increasing, it means that the consumer is deriving more satisfaction from consuming additional units of the good. In this case, the marginal utility becomes positive as each additional unit adds to the overall satisfaction.

Option 4 states that when total utility is maximum, marginal and average utility are equal to each other. This statement is incorrect. The correct relationship is that