"Who are they?" should work, from a gender-neutral perspective. Using "they" as a third-person singular pronoun in English is particularly appropriate in traditional usage when speaking about an unfamiliar person, and also increasingly common even when referring to a known person (if they do not identify as a man or woman, or if they otherwise prefer a gender-neutral pronoun).
As long as we realize that this is strictly about a single person. Because the plural version is "Kik ők?".
Other than that, "ő" is the actual third person pronoun, so, translating it to "Who is this/that?" is not necessarily correct, but that is debatable. The close translation of those is "Ki ez/az?".
I believe GrowingViolet has right. In English (at least in the US) the gender can be neutralized by using "they" instead of "he/she".
For example, someone is knocking the door but by the time you open it nobody is there. It is correct to say that "Who is there? ... Hmm, no one. They must have been gone."
That is all true, but "Ki ő?" would be used when talking about a specific person. For example, you see somebody at a party whom you don't know. Or you are looking at some old family photo and see a person you don't recognize. Pointing at the person, you ask: "Ki ő?" - "Who is he/she?" You would not say "Who are they?", as you are clearly asking about one single person. Maybe you would say "Who is that person?". But the Hungarian equivalent of that question is "Ki az a személy?".
Above, koszeggy wrote that "in English (at least in the US) the gender can be neutralized by using 'they' instead of 'he/she.' " What is really occurring in the US is that the singular "they" is often used, but primarily colloquially. Edited English, as in The New York Times, still strictly considers "they" as plural only; the Times routinely corrects its writers. This is a centuries-old debate, but the singular "they" seems never to have been as thoroughly well accepted as the singular "he" or "she," or even "he and she"--or "she and he." In edited English, if one wrote "they" in reference to a single person, many would become confused and begin looking for a plural antecedent.
And thanks to vvsey for answering my suspicion elsewhere on this page about whether "ő" is used only for "he" and "she" and never for "it." Hungarians indeed use it only for "he" and "she," if I rightly understand what he or she wrote (speaking of the puzzle!).
And a nice solution we Americans use when referring to a hypothetical he or she is to equally alternate the use of the two words.
Yeeeah, especially in your speech. O tempora o mores x) Anyway, your suggestion is too complicated. In real world, you and the person you're asking would know gender from the context. So, no need for complication here.
On the other hand, I don't understand why "Who is it?" won't be accepted :/ Report!
Those are two sentences, but the Hungarian is just one sentence.
If you look carefully at the error message, it will probably say something about “correct solutions” — note the -s on “solutions” that marks it as plural.
Pick one or the other, but don’t write both at once.