my answer, "which one is the bedroom?", was graded incorrect. However, I understood the question as if it was asking to clarify which one was the bedroom (or dormitory) out of other rooms. Hopefully I make sense... :)
Perhaps it was marked incorrect because "dormitório" is not "bedroom", and not because of your phrase construction...?
I think "bedroom" is a perfectly acceptable translation for "dormitório" or "quarto".
Depends on how precise you want to be. For learning purposes, I think you should be as much as you can. "Dormitory is a specific type of bedroom, destined for a bunch of people, as in a boarding school, a barrack or something like that.
Does this sentence really mean "Which dormitory is it?" Shouldn't it rather be "Which one is the dormitory?" I am a bit unsure how these two would be expressed and whether the given answer is correct.
One more for "Which one is the bedroom". It sounds like a correct translation of the Portuguese sentence.