The word flunked is definitely colloquial.
Colloquial words are those which are considered to be informal, words you wouldn't normally used in a formal conversation, but rather when you are talking to your friends, for example. In formal situations, you'd have to use their formal counterparts.
If you can choose more than one option, I'd also pick trig, as it is short for trigonometry, but if there is only one correct option, then it's definitely flunked.