To the right of each quiz, there is a check-box which shows the student whether they have completed that assignment. Usually, it has a blue check mark in it. My student today asked me why there was a red X there. I've never seen that before. Can anyone help me with that? see the screenshot.
It means they did the quiz but did not pass it - there must be a pass grade for the quiz. See the section on Quiz in the documentation Activity completion
That's it. Thanks for sharing.