1) If you are populating question on same page then only it’s better to retain the values in Viewstate till the last question answer attempted.
2) If you want to restore the all selection once user quit from the quiz then no need to save the question answers till user attempt the all the question. In last you can save all the values which you have retain in ViewState if it’s on same page.
3) Its based on your Question Answer Populating and retaining logic as how you are doing.