"Do you take a walk often?"
Translation:Ты часто гуляешь?
So what is the difference between "Ты часто гуляешь?"(which is listed as correct answer) and "Вы гуляете часто?" (which was not accepted).
No, you can just say "Часто гуляешь?", but I wouldn't start a new topic this way. I would rather use this sentence if walking was already mentioned in the conversation somehow.