Final answer:
The hypothetical imperative is based on 'purpose', and the correct answer to the question is D. purpose; X; Y. It serves as a conditional command suggesting that to achieve X, one should do Y.
Step-by-step explanation:
The hypothetical imperative is a concept derived from the philosopher Immanuel Kant. A hypothetical imperative is a commandment of reason that applies only conditionally, stating that if one wishes to achieve a certain goal, one should take specific actions. It indicates that an action is good for a specific purpose.
Therefore, the answer to the student's question is D. purpose; X; Y. Hypothetical imperatives are based on the purpose they serve, and they imply a means to achieve a particular goal, which is in contrast to categorical imperatives that are universal and must be obeyed regardless of our desires or ends.