The word taught being "participant(e)(s)", the preferred translation must have the English translation for that word, so that when you get it to translate it back to French, you can have a hint of the word you should use.

For your information, "contestants" can back translate to "concurrent(e)s" (usually in sports) or "compétiteurs/compétitrices" (sports or other types of contests).

But that is marked wrong, too

Actually the exact translation is "competitors" because it applies to physical competition. "contestants" usually is used for participants in a game (like the ones on TV).

Something else might have been wrong, because "contest" is accepted.