"Mister, why is the car fast?" is a perfectly valid answer. There's no context to this sentence so without 들, it can be either.
So suggest it! They need as many suggestions as they can for possible answers
The use of 는 marks "cars" as a topic. Therefore it makes more sense to word it as "Mister, as for cars, why are they fast?"
i heard that i should refrain from saying 아저씨 to men who are young.. it means like "old man" or something?? jin (from bts) said something like that