Try some of these:
Ask for clarification-you can repeat or paraphrase the question,or pose question of your own to get more information,such as "do you mean?"or "could you give an example of?"
You could try to steer your response to something you have read or are confident about, such as "i'm not sure about that, but what i do think is" or "i haven't thought about that particular point, but something related to it is"
Try opening the question out to the rest of the group by asking "what do people think about this?"or "what are other people's thoughts on?