@lily If you fix a question after students have started a quiz, the corrected version will only appear to students who haven't yet started the quiz. It should also appear to students who get a second attempt, if the question was fixed before they started the next attempt and "Build on Last Attempt" isn't selected. 

In general, editing a quiz, even for minor fixes, after students have started a quiz, is not recommended since it is difficult to track who got which version of the quiz. Instead, I recommend waiting until all students have completed the quiz and then use the Regrade tool to accept additional correct answers for the question. This works much better, in my experience, since the regrade is automatically applied to all submissions that had that particular question.