Final answer:
In economics, total utility and marginal utility are related concepts that measure consumer satisfaction. When marginal utility is zero, total utility can still be rising but at a declining rate.
Step-by-step explanation:
In economics, the concept of utility refers to the satisfaction or pleasure that a consumer derives from consuming a good or service. Total utility is the overall satisfaction a consumer gains from consuming a certain quantity of a good or service. Marginal utility is the additional satisfaction gained from consuming one more unit of a good or service. When the marginal utility is zero in panel (b), it means that the consumer is no longer receiving any additional satisfaction from consuming additional units of the good.
In panel (a), total utility can be rising but at a declining rate. This means that as the consumer consumes more units of the good, the total satisfaction is increasing, but at a decreasing rate.