The correct answer is: c) Terracotta.
A kalah is a water pot found in Assamese households. It is often made of terracotta, which is a type of unglazed pottery made from clay. Terracotta is a durable material that is resistant to water and heat. It is also a natural material that is environmentally friendly.
Copper and brass are both metals that are often used to make water pots. However, they are not as durable as terracotta and are more likely to corrode. Glass is also a durable material, but it is not as common as terracotta for making water pots. This is because glass is more expensive and can break more easily.
In conclusion, the correct answer is c) Terracotta. Kalahs are often made of terracotta because it is a durable, natural, and environmentally friendly material.