A student (high school age) asked me if "that you are least proud of" in What have you done that you are least proud of? is an interrogative clause or a relative clause. My answer was based on intuition rather than solid linguistic background.
My answer: it depends. If the clause is asking about something that the speaker doesn't know about, it's interrogative. If the clause is actually factual, it's a relative clause. In other words, the speaker already knows what the answer is and simply wants the other person to acknowledge this answer in the conversation (that is, the speaker is not seeking the answer to what he doesn't know).
Is my intuition correct, or is there a better way of analyzing such sentences?