What types of chlorine are commonly used in water treatment?

The correct choice highlights the forms of chlorine that are most commonly utilized in water treatment processes. Chlorine gas is an effective disinfectant and is often used for large-scale treatment facilities due to its potency and efficiency in killing bacteria and pathogens in water. Chlorine solutions, such as sodium hypochlorite (liquid bleach), are also widely used since they are easy to handle and can be dosed accurately. Granular forms of chlorine, like calcium hypochlorite, are convenient for handling and dosing in smaller operations or for shock chlorination.

The other options include forms of chlorine that may not be typically used in drinking water treatment. For example, “foam” does not represent a distinct, effective form of chlorine for disinfection. Similarly, terms like "powder" or "solid" could lead to confusion, as while some compounds can be in these forms, they are less common in standard water treatment practices. Thus, the combination of gas, solution, and granular accurately reflects the commonly accepted and effective methods of applying chlorine in water treatment.