If you would only play one note then it wouldn't be music but rhythm or poetry.

Over the C chord if you would play the root note it would sound strong and resolved like the end ( maybe happy ending ) of a story, if you would play the 3rd ( E ) it would create tension that could go either way, more like a question to which you don't know the answer yet and if you would play the fifth ( G ) then it want's to resolve to C but since you are only aloud to play one note then this would keep you at your hunger because it can't resolve and would cost you your sleep at night LOL

Just my 50 cents Herman