Final answer:
If total utility decreases with the consumption of an additional unit of a good, the marginal utility is decreasing. This is illustrated by the economic principle of the law of diminishing marginal utility, where each additional unit consumed provides less utility than the previous one.
Step-by-step explanation:
If a consumer's total utility decreases with the consumption of one additional unit of a good, we know that marginal utility must be decreasing. Marginal utility is the additional satisfaction or utility that a consumer receives from consuming an additional unit of a good or service. In economics, we often observe the law of diminishing marginal utility, which tells us that as a consumer consumes more units of a good, the additional utility gained from each new unit tends to decrease.
For instance, consider José's experience with collecting T-shirts. The first T-shirt brings him significant satisfaction, adding 22 utils to his utility, while the fourth T-shirt, which he may only wear when his other clothes are dirty, contributes a lesser additional utility of just 18 utils. This exemplifies the concept that each subsequent unit adds less to total utility, thus indicating that the marginal utility is diminishing.