"Who is this person?"
it is long, but it's also more formal. You could certainly say この人は誰ですか, or even just だれですか, but those would be for a much more casual situation. If you were in a business meeting, for example, using 人 instead of 方 or 誰 instead of どなた would be considered extremely rude. It all depends on the circumstances. (And as I tell my students, it's a pretty good rule of thumb: the more syllables a phrase has, the more polite it is!)
A lot of questions seem to be missing a certain point to learning a language (though I'm no expert, and Duo certainly has its issues): There are multiple ways to say something. From my understanding, asking「どなたですか」verses this lesson's answer would functionally convey the same idea, but they aren't the same question -- not literally, at least. If I ask you to write down the words "Who is this person?", submitting the answer "Who is this?" would obviously not get a 100%. This is a rudimentary language course, not a choose-your-adventure story.
In Japanese, personal questions incouding asking someone's name are somewhat rude by tradition so, by and large, i'd recommend erring on the side of politeness. あの人誰(anohito dare) is something you could say but comes across as speaking both to a friend and about someone you have very little respect for. Hellonative could also be helpful for natural Japanese