"Who is this person?"
In Chinese, the position of the question word, like 谁, 什么 is supposed to be replaced in the answer. When used with verb "be", it is generally put after "be".
(Note: This indicates that Chinese can have multiple question words in one sentence. You just need to fill more holes in your answer.)
这个人是谁？(This man is who?) or 谁是这个人？(Who is this man?) are both valid. But the latter is a bit weird.
After replacement, it would be like:
这个人是我。(This man is me.) or 我是这个人。(I am this man.)
If you want to say "Who is he?", it would be 他是谁？(He is who?)
In English, questions have to begin with a question word, that's why both start with "who". But in Chinese, you replace the word you're asking for by the question word.
谁是你的爸爸 - lit. WHO is your dad? - answer: 他是我的爸爸 - HE is my dad.
这个人是谁 - lit. This person is WHO? - answer: 这个人是我的爸爸 - This person is MY DAD.
There must be some bug, because I wrote 这个人是谁? which is the given correct answer, and it marked it incorrect! Or was the problem that I had a question mark?