There is no C.M.S. that I know which would allow you to enter fractions like the one you mention or complicated mathematical expressions in any modest one line of a drop-down menu. What all systems do when the matching options are large objects (images for example) or expressions that are different from a simple one-liner ASCII text (like the math expression that you are trying to type) is to list the options in a reference table with letters as labels (a, b, c, ...), and then give those letters as the matching options of the dropdown menu. I appended below some examples (from different CMS's) on how such questions are generally adapted to use "a, b, c, ..." labels for matching questions.
Note: There are some "Math" UNICODE characters that can help you to enter some Math symbols in the dropdown menu, but the list is finite and for certain will not include the variety of fractions that you need. This is the link that Jeff Forssell kindly gave me http://www.atm.ox.ac.uk/user/iwi/charmap.html (look for the list #22 that contains Math symbols)
Hope this helps...