Have you removed any of the default captions from your quiz questions? I refer to the Question Title caption that tells what type of question is is (True/Fales, Multiple Choice) or the Question text caption that holds the question.
If so, that can cause malfunctioning quiz questions.
Your project might also have become corrupted. To test this assumption, create a new blank Captivate project file and insert a single multiple choice quiz question. Change NOTHING about the question slide, then test in HTML5 output.
Do you see the same issue even in a new blank project? If so, you may have a problem with your Captivate installation. If not, then your current project may be corrupted.