The correct answer is: A. Cracking and warping of bricks.
Excess silica in brick earth can cause the bricks to crack and warp during firing. This is because silica is a non-plastic material, meaning that it does not have the ability to be molded or shaped. When silica is heated, it expands and contracts at a different rate than the other materials in the brick earth, which can cause the brick to crack. Additionally, silica can also cause the brick to warp, as it can absorb water and swell.
Option B is incorrect because excess silica does not cause loss of cohesion. Cohesion is the force that holds particles together, and it is not affected by the amount of silica in the brick earth.
Option C is incorrect because excess silica does not enhance the impermeability of bricks. In fact, excess silica can actually make bricks more porous, as it can absorb water and swell.
Option D is incorrect because excess silica does cause cracking and warping of bricks.