In substitution type of questions, some particular objects are assigned code names. Then a question is asked that is to be answered in the code language.
If wall is called window, window is called door, door is called floor, floor is callled roof and roof is callled ventilator, what will a person stand on ?
A person will stand on the 'floor' and 'floor' is called 'roof'.
So, a person will stand on the 'roof'.