The fill in the blanks for Quizzes.next doesn't have an html editor or a math editor. However, while tinkering tonight, it turns out that it is very easy to add a subscript. For example, S naught or S_0 in classic doesn't work but it is possible to use the following:
I made this a discussion in case others have discovered other hidden ways to add math to fill-in-the-blanks. For example, the following didn't work for me (no square root) and maybe someone can explain what I did wrong.
I look forward to discovering more!