— Who is absent from the lecture today? —_____________you ask? Peter, of course. A. Would B. Can C.

— Who is absent from the lecture today? —_____________you ask? Peter, of course. A. Would B. Can C. Could D. Need

The correct answer is B. Can.

So the sentence would be:
— Who is absent from the lecture today?
— Can you ask? Peter, of course.

"Can you ask?" is a natural way of confirming or suggesting that someone can be asked, which fits the context of the conversation.