The correct answer is: Salt.
Salt is the main raw material used in the soda ash industry in Gujarat. It is used to produce sodium carbonate, which is a key ingredient in many products, including glass, detergents, and paper.
Lignite is a type of coal that is not as hard as bituminous coal or anthracite coal. It is used to generate electricity and to produce coke, which is used in the steel industry.
Limestone is a sedimentary rock that is composed of calcium carbonate. It is used to produce cement, lime, and other building materials.
Gypsum is a mineral that is composed of calcium sulfate. It is used to produce plaster, drywall, and other building materials.
Salt is a mineral that is composed of sodium chloride. It is found in seawater and in salt deposits on land. Salt is used in food, as a preservative, and in many industrial processes.