It's not D because a root is something like "sub" in "subway" which means "under, below" or "ped" in "pedestrian" which means "foot." "-ly" has no particular meaning.
B, or a preposition, I believe, renames the subject. "-ly" does not rename any subject in particular, so that is also false.
A is not correct either simply because a prefix is a morpheme that goes at the beginning of a word. "-ly" is supposed to go at the end of a word.
So the correct answer is C, or suffix :)