The correct answer is: A. 20 to 25 mm.
Bituminous carpet is a type of roofing material that is made from a mixture of asphalt, aggregate, and fibers. It is applied to roofs in a thin layer and then rolled out to create a smooth, watertight surface. The thickness of bituminous carpet varies depending on the type of roof and the climate in which it is installed. In general, bituminous carpet is applied in layers that are 20 to 25 mm thick.
Option B is incorrect because bituminous carpet is not typically applied in layers that are 50 to 75 mm thick. Option C is incorrect because bituminous carpet is not typically applied in layers that are 75 to 100 mm thick. Option D is incorrect because bituminous carpet is not typically applied in layers that are 100 to 120 mm thick.