Correct option is (a) 60 cm
The best explanation: A parapet is a barrier which is an extension of the wall at the edge of a roof, terrace, balcony, walkway or other structure. The word comes ultimately from the Italian parapetto. The German equivalent Brustwehr has the same meaning. Where extending above a roof, a parapet may simply be the portion of an exterior wall that continues above the line of the roof surface, or may be a continuation of a vertical feature beneath the roof such as a fire wall or party wall.