The correct answer is C. Lime.
Lime is a chemical compound with the formula CaO. It is a white, powdery, or granular solid that is produced by heating limestone. Lime is used in a variety of industries, including construction, agriculture, and wastewater treatment.
In wastewater treatment, lime is used to increase the speed of sedimentation. Sedimentation is the process of particles settling out of a liquid. When lime is added to wastewater, it reacts with the organic matter in the water to form a precipitate. This precipitate is heavier than the water and sinks to the bottom of the tank, where it can be removed.
Sulphuric acid, copper sulphate, and sodium permanganate are not commonly used to increase the speed of sedimentation of sewage. Sulphuric acid is a strong acid that can corrode equipment and damage pipes. Copper sulphate is a toxic metal that can be harmful to aquatic life. Sodium permanganate is a strong oxidizing agent that can react with organic matter to produce harmful compounds.
Therefore, the correct answer is C. Lime.