All of the options you provided can contribute to the relevance of a question or comment in a discussion, but if I had to choose the most fundamental aspects, I would say:
A) It is clearly connected to the topic.
B) It follows the rules set by the group.
C) It responds to a question.
D) It explores an idea.
Among these, A is the most direct way to ensure relevance, as a comment or question must be connected to the topic at hand. Options B, C, and D also enhance relevance but are more contextual or procedural. Therefore, all of these factors work together to help maintain a focused and productive discussion.