ANS:D - alumina.

Alumina plays a major role in brick. It acts as a cementing material in raw brick. Brick clay is plastic due to the presence of alumina. This plasticity ensures that bricks can be molded. An excess amount of alumina in clay may cause the bricks to shrink, warp, or crack on drying and burning as any other cementing material. The Percentage content of the brick does not make the brick the main constituent. Alumina is the main constituent of every kind of clay. The only thing that matters is function or role similarly. So, here alumina is correct.