The bedroom is at the rear side of this house.

The correct answer is B. back.

The bedroom is at the rear side of the house. This means that the bedroom is located at the back of the house, away from the front door. The other options are incorrect because they do not describe the location of the bedroom relative to the front of the house. Option A, front, is incorrect because the bedroom is not located at the front of the house. Option C, upper, is incorrect because the bedroom is not located on the upper floor of the house. Option D, different, is incorrect because it does not provide any information about the location of the bedroom.