Which of the following compounds is used to help treat water?

Chlorine is a widely used disinfectant in water treatment processes. It effectively eliminates a broad spectrum of pathogens, such as bacteria, viruses, and protozoa, thus ensuring that water is safe for human consumption. Its use in water treatment is critical for public health, as it helps to prevent waterborne diseases that can be transmitted through untreated water. Chlorine can also help control taste and odor concerns, making water more palatable.

Other compounds mentioned, like sodium chloride, calcium carbonate, and iron sulfate, have their roles in water treatment but are not primarily used for disinfection. Sodium chloride may be involved in processes such as ion exchange or softening, calcium carbonate can play a role in adjusting pH or hardness, and iron sulfate is often used for coagulation and removal of impurities rather than for disinfection. Thus, chlorine stands out as the essential compound specifically targeting pathogen removal in water treatment.