A biased question is favoring one side over the other. For example, in your problem, the first one,The principle wants to find out if students are satisfied with the school cafeteria. I would pick letter c as the unbiased sample because the principle is asking the students who have to eat the lunch if they like it or not. but i dont know if its completely right because the way you stated you question because i dont really know what your asking.Hope i helped though. Good luck on your paper!