Which substance is often used to treat drinking water for disinfection?

Chlorine is commonly used in water treatment for disinfection due to its ability to effectively kill bacteria, viruses, and other pathogens that can be harmful to human health. It works by breaking down the cell walls of microorganisms, rendering them inactive and preventing the spread of waterborne diseases. Chlorination not only disinfects the water but also helps in maintaining a residual effect in the distribution system, providing ongoing protection against any potential contamination after treatment.

Sodium chloride is primarily used for water softening and various industrial applications but does not serve a direct disinfection purpose. Calcium carbonate and magnesium sulfate are typically used to adjust water hardness and balance mineral content rather than providing disinfection. Thus, chlorine stands out as the key substance for disinfecting drinking water, making it the right answer to the question.