I think it should be accepted. When you don't mean some particular cars and are asking "Where are cars (in general)? - In the cities"
I guess I need an English native speaker because, you know, Ukrainians often drop articles where we do need them (e.g. "I have cat" instead of "I have a cat") and I have to pay attention to that when I speak, so now that I want to drop it here I feel uncomfortable :D
This is a course to teach Ukrainian, not English, so I'm not sure this is the best place to hash out the finer points of articles, which, as many people have pointed out, don't exist in Ukrainian. The fact is, "Where are cars?" sounds very odd in English. Де машини? does not sound odd in Ukrainian. So despite the fact that a word "is being added," the most natural translation is "Where are the cars?"
Why shouldn't they both be correct? In Ukrainian, there are no articles. If I say "Я їм піцу" this might mean one of the three things: "I eat a pizza", "I eat the pizza", and "I eat pizza". If we have to specify, we might use "some" (indefinite), "this"/"that" (definite), "a slice of". But usually it is obvious from the context.