Which type of chlorine is commonly used for disinfection in water treatment?

All of the options listed—chlorine gas, sodium hypochlorite, and calcium hypochlorite—are commonly used in water treatment for disinfection purposes. Chlorine gas is the most basic form of chlorine and is effective at low concentrations, making it a traditional choice for large-scale disinfection in water treatment plants. Sodium hypochlorite, often found in household bleach, is a liquid form that is easy to use and transport, and it is favored for smaller facilities or situations where handling gas is impractical. Calcium hypochlorite, though also a solid, is used primarily in granular or tablet form and is effective in both chlorination and disinfection processes. Its stability and effectiveness make it suitable for different water treatment circumstances. Each of these chlorine compounds plays a vital role in maintaining water quality and safety by targeting and eliminating pathogens present in the water supply. Therefore, the inclusion of all three forms makes the option of "All of the above" the right choice.