"Who is that?"
If you do not know the gender of the person, you say, "Who is that?" or "Who is it?" For example: You are shown a faded photograph, and you think you recognize your friend, you might ask, "Who is that?", to be sure. If someone knocks at the door when you are not expecting anyone, it is common to ask, "Who is it?" before you open the door.
There is no "that" in this sentence. When I make the slightest mistake in the English translation Duo freaks out, but Duo itself always makes wrong translations. In Spanish-English it is even worse. I thought "who is that" is translated as "su kim", but "o kim" would be "Who is it" . But I don´t have a clue because I just started a few days ago with learning Turkish. Inspite of that, I have the feeling, that the Egnlsih translations are very often not very precisely.