In every question in your quiz you will see a book mark icon. Click on the icon would add the question to a review list. The questions added in review list have a different book mark icon to denote that they are in the bookmark list.
Besides the questions list you will see a book mark icon , click on that would only show the questions which are bookmarked by you. The pagination would only show the bookmarked of question pages. Click again on icon to come out of the list.
Click on bookmark icon again to un-list the question from review list.
At the time of submission if there are any book marked questions you will be notified about it.
The above is just a notification, you can chose to go ahead and submit the quiz, without issues.