"Good, thank you."
That largely depends on where one comes from; in the U.S. it's not unusual nor uncommon to say "I'm good, thanks". Considering how many times this platform beats an expression into one's head, then suddenly introduces a new word or expression out of nowhere that is suddenly acceptable, then the path should be widened to accept more phrases. Not just one.
...and that's the problem. Good is an adjective, so if you say "I am good" you are effectively saying "I am not naughty". Until only a decade or so ago, we always used to say "I am fine" or "I am well" when asked about our condition (how are you) and many English speakers still do, rather than describing our behaviour in response.
The statement one assumes is the answer to "how are you?' If so it is grammatically wrong to answer with the adjective "good" but with "well" the grammatically correct adverb. So the French answer "Bien," merci (Well thank you) is fine, however "Good" the first word should be substituted with "Well". Thanks DUOLINGO.
If Duo wants a translation of it's own poor English grammar (assuming it is a response to how one is doing, which has been the running theme), it should accept "bon" or "bonne" rather than the grammatically correct response. If Duo wants correct grammar, it should hold its own end of the bargain. I expect better.