The correct option is A Bungalow
A small house with all the rooms one one floor is a Bungalow, Option A. Option B, cottage means a small house, usually in countryside. Option C, flat is a set of rooms for living, usually on one floor in a large building. Option D, castle is a large strong building built in ancient times to protect people from attack. So, options B, C and D are incorrect and the correct option is Option A, Bungalow.