Final answer:
Kant believes good will is the only intrinsically good thing, based on the decision to perform moral duties, while utilitarians like Mill hold that happiness or pleasure (utility) is intrinsically good, focusing on consequences that promote happiness.
Step-by-step explanation:
Kant and utilitarians like Mill disagree fundamentally about what is intrinsically or unconditionally good. The correct answer to the student's question is neither A, B, C, nor D. Instead, Kant believes that good will is the only intrinsically good thing because it reflects a person's decision to fulfill their moral duties, irrespective of the outcomes of their actions. This view is grounded in his deontological ethical framework which emphasizes duty and the categorical imperative. In contrast, utilitarians hold that happiness or pleasure is intrinsically good, which they define as utility. Utilitarianism is a consequentialist theory, which means it is the consequences of actions, particularly those that promote happiness or utility, that are morally relevant.