Right option is (a) 50 %
The best I can explain: Carpet Area is the area enclosed within the walls, actual area to lay the carpet. Built up Area is the carpet area plus the thickness of outer walls and the balcony. Super Built up Area is the built up area plus proportionate area of common areas such as the lobby, lifts shaft, stairs, etc.
In architectural, construction, and real estate, floor area, floor space, or floor space is the area (measured as square feet or square metres) taken up by a building or part of it. Gross floor area (GFA) – The total floor area contained within the building measured to the external face of the external walls.