The correct answer is: A. To impart plasticity
Alumina is a type of clay that is used in brick earth to impart plasticity. Plasticity is the ability of a material to be molded or shaped without breaking. Alumina helps to make brick earth more workable and easier to form into bricks.
Option B is incorrect because alumina does not make bricks durable. Durability is the ability of a material to withstand wear and tear. Alumina does not provide any additional durability to brick earth.
Option C is incorrect because alumina does not prevent shrinkage. Shrinkage is the reduction in size of a material that occurs when it dries or cures. Alumina does not prevent shrinkage in brick earth.
Option D is incorrect because alumina does not make bricks impermeable. Impermeable materials are not able to allow water or other liquids to pass through them. Alumina does not make brick earth impermeable.
In conclusion, the main function of alumina in brick earth is to impart plasticity.