Duo never pays any attention to punctuation. That makes me lazy, but it makes things faster on mobile...
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
Додайте "Where are cars?". Подивіться підказку, там пропонується Where are (the) cars?, тобто the не є обов'язковим. 24.10.15
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 are both 'where are cars?' and 'where are the cars?' correct here? although neither is possessive, and both are plural
Ukranian has no articles. You can tranclate article «the» like «це» but it's really wrong. Because it's impossible to translate something what has no equivalent. So it's reason why both answers are acceptable.
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.