That's what I had too and it was marked wrong. Please explain why or whether it should be reported.
"Kik veszik" is definite 3 person plural, right? If so, is "Who are buying the cars?" also a possible translation for this sentence?
You are correct. Who, generally, is treated as singular, even when it "makes sense" or "is meant" to be multiple people.
So who are is grammatically correct, but is unnatural. It is only natural when the number of people referred to by who is explicitly more than one (by grammar or plural noun, not implicitly). Ex. Who is on the basketball team? vs Who are the people going to the game?
This also leads to a weird situation where you may switch to who are based on another sentence. Lots of people are buying cars and trucks. Who are buying the cars? The people who aren't buying the trucks. In this example, the previously odd-sounding Who are buying the cars? should suddenly sound OK.
Thanks! That answer was marked as incorrect. But I'll report it next time I see it.