It would be valuable to flag questions to go back to them as you are taking a quiz, and when you do have the circle indicate that it was flagged so you can directly navigate to them.
Students can flag questions while taking a quiz. Flags appear on the review screen and students can navigate directly to those questions from the review screen. This feature would add an additional flag to the circle navigation so that students can see what questions are flagged without going to the review screen.
We are currently reviewing how the circle indicators for quizzes work. On most smart phones, there is only room for 6 or 7 circles, which makes navigating using those circles less effective. We will consider this functionality in future iterations of that navigation in quizzes.