A small house with all rooms on one floor

Answer: Option A

One word substitute is Bungalow.

Bungalow: a low house having only one storey or, in some cases, upper rooms set in the roof, typically with dormer windows.
Cottage: a small house, typically one in the country.
Flat: having a level surface; without raised areas or indentations.
Castle: a large building, typically of the medieval period, fortified against attack with thick walls, battlements, towers, and in many cases a moat.