Is there a setting so that FormReturn alerts me if someone has not filled in an answer for a question?

Yes, you can use a mark aggregation rule to do it. For example:


That says if B (and only B selected) is the correct answer, add one mark, if nothing is selected, add 99999, for everything else do nothing.

Looking at the Form Score column in the captured data should quickly alert you to a problem that needs manual reprocessing.