In the context of chlorination, the effectiveness and activity depend on the chemical species involved. Chlorination is a process where chlorine is used to disinfect water by eliminating pathogens and impurities. Let's analyze each option:
Hypochlorite ion: Often present as sodium hypochlorite, it is used for disinfection but is not as active as hypochlorous acid in its pure form.
Hydrogen chloride: Primarily used as an acidifying agent, it does not act as a disinfectant in chlorination.
Hypochlorous acid: This is the most effective form of chlorine in water treatment, offering strong antimicrobial activity. Its efficiency is due to its ability to penetrate microbial cell walls easily.
Chloride ion: By itself, it does not exhibit disinfecting properties.
The correct answer is Hypochlorous acid, as it is the active form that provides significant disinfecting power in water treatment processes.