Okay, let's tackle this question about the disinfecting action of chlorine in water chlorination. The question is from 2018, so I need to make sure the answer is still current. The user didn't provide the options, but based on common knowledge, the main active component here is likely hypochlorous acid (HOCl).

First, the core concept here is the mechanism of chlorine's disinfectant action. Chlorine is commonly used for water disinfection, and when added to water, it reacts with water to form hypochlorous acid. Hypochlorous acid is a strong oxidizing agent that disrupts microbial cell membranes and denatures proteins and enzymes, leading to cell death.

Now, the correct answer should be the option that mentions hypochlorous acid. The other options might include things like hydrochloric acid, sodium hypochlorite, or maybe other compounds. Let me think: when chlorine gas is dissolved in water, the reaction is Cl2 + H2O → HOCl + HCl. So hypochlorous acid (HOCl) is the active disinfectant, not hydrochloric acid (HCl), which is a byproduct but not responsible for disinfection. Sodium hypochlorite (NaOCl) is a common source of chlorine used in chlorination, but the actual active form is HOCl when dissolved in water.

For the incorrect options, if they include hydrochloric acid, that's a distractor because it's a byproduct but not the active disinfectant. If there's an option for sodium hypochlorite, that's the source but not the active form in water. Another possible distractor could be hypochlorite ion (OCl-), which is less effective than HOCl in acidic conditions. The hypochlorous acid is more effective because it's a neutral molecule and can penetrate cell membranes more easily than the ionic form.

Clinical pearl: Remember that hypochlorous acid is the active disinfectant, and its effectiveness depends on pH. Lower pH (more acidic) increases the proportion of HOCl compared to OCl-, making disinfection more effective. This is a key point for exams like NEET or USMLE.

So putting it all together, the correct answer is the option that states hypochlorous acid (HOCl) is responsible for the disinfecting action.

**Core Concept**
Chlorine's disinfecting action in water chlorination relies on its conversion to **hypochlorous acid (HOCl)**, a potent oxidizing agent. HOCl disrupts microbial cell membranes, denatures proteins, and inactivates enzymes, effectively killing pathogens like bacteria and viruses. This reaction occurs via the equilibrium: **Cl₂ + H₂O ⇌ HOCl + H⁺ + Cl⁻**.

**Why the Correct Answer is Right**
Hypochlorous acid (HOCl) is the active disinfectant formed when chlorine gas reacts with water. It is a weak acid but highly effective due to its ability to penetrate microbial cell walls. Its oxidizing properties damage cellular components, including DNA and enzymes, leading to microbial death. The proportion of HOCl versus its dissociated form (OCl⁻) depends on pH: lower pH favors HOCl, enhancing disinfection efficacy.

**Why Each Wrong Option is Incorrect**
**Option A:** If

✓ Correct Answer: D. Hypochlorous acid