The new Bookmark a question option allows users to mark certain questions in the question list when they take a quiz. There can be several reasons to bookmark a question. A student might want to go back and check the answer before submitting the quiz. 

To bookmark a question, go to the question and click the  Bookmark button next to the question number. The flag icon will appear in the question list next to the selected question. 

When you are ready with the answer choice, you can unflag a question. To do it, go to the question and click the Bookmark button again. 

Bookmarking is available when answers are submitted all at once. When questions are submitted one at a time, there is no reason to bookmark a question since you get an immediate feedback after you answer it.

To choose the Submit all at once mode, open the Quiz Properties and go to the Navigation tab.