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... :)
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.