The amount of chlorine used by organic matter in water is a portion of the...

The correct answer is based on the principle of how chlorine interacts with various components in water, particularly organic matter. Chlorine demand refers to the quantity of chlorine that is required to achieve a desired disinfection level after accounting for various reactions that chlorine undergoes.

When chlorine is added to water, it reacts not only with pathogens but also with organic matter present in the water. This reaction consumes chlorine, which means that a certain amount of chlorine is 'used up' by these reactions. Therefore, chlorine demand effectively captures this consumption process, representing the amount of chlorine that must be added to ensure that sufficient residual chlorine remains available for disinfection after these initial reactions.

Chlorine residual, on the other hand, measures the amount of chlorine that remains in the water after these reactions have taken place, while chlorination efficiency would refer to how effectively chlorine serves its purpose in the treatment process. Free chlorine pertains specifically to the portion of chlorine that is not combined with other substances (like organic matter) and is available to disinfect effectively. Thus, all these factors contribute to understanding water treatment, but in this context, chlorine demand is the factor that accounts for the chlorine used by organic matter.