**Core Concept**
Chlorination is a disinfection process used to kill bacteria, viruses, and other pathogens in water and other substances. In water treatment, chlorine is added in various forms to inactivate microorganisms.

**Why the Correct Answer is Right**
The active form of chlorination is hypochlorous acid (HOCl), which is a potent disinfectant. Hypochlorous acid is formed when chlorine (Cl2) reacts with water (H2O). This reaction is catalyzed by the enzyme catalase, which is present in most living organisms. HOCl is a strong oxidizing agent that denatures proteins, disrupts cell membranes, and inactivates enzymes, ultimately leading to the death of microorganisms.

**Why Each Wrong Option is Incorrect**
**Option A:** Chlorine gas (Cl2) is the raw material used for chlorination, but it is not the active form of chlorination.
**Option B:** Hypochlorite ions (OCl-) are also formed during chlorination, but they are less effective disinfectants than hypochlorous acid.
**Option C:** Chloride ions (Cl-) are present in the solution but do not play a direct role in the disinfection process.

**Clinical Pearl / High-Yield Fact**
Remember that hypochlorous acid is a weak acid and has a very short half-life, which is why it is often generated in situ during the chlorination process to maintain its effectiveness as a disinfectant.

**Correct Answer: B. Hypochlorite ions are also formed during chlorination, but they are less effective disinfectants than hypochlorous acid.