**Core Concept**
The disinfectant action of chlorine is primarily mediated through the formation of hypochlorous acid (HOCl), a potent antimicrobial agent that disrupts cellular membranes and inactivates enzymes essential for microbial metabolism.

**Why the Correct Answer is Right**
Hypochlorous acid (HOCl) is formed when chlorine reacts with water to produce a mixture of HOCl and hypochlorite ions (OCl-). HOCl is responsible for the majority of the disinfectant action of chlorine due to its ability to denature proteins, inactivate enzymes, and disrupt the cell membrane of microorganisms. This results in the death of microorganisms and the inactivation of viruses.

**Why Each Wrong Option is Incorrect**
**Option A:** Chlorine gas is not the primary form responsible for the disinfectant action of chlorine. While it is a potent irritant and can cause respiratory distress, its antimicrobial properties are not as effective as those of HOCl.
**Option B:** Hypochlorite ions (OCl-) are formed when chlorine reacts with water, but they are less effective antimicrobial agents than HOCl. OCl- is primarily responsible for the bleaching action of chlorine.
**Option C:** Chlorine dioxide (ClO2) is a disinfectant gas that is used to control microbial growth, but it is not the primary form responsible for the disinfectant action of chlorine.

**Clinical Pearl / High-Yield Fact**
When using chlorine as a disinfectant, it's essential to maintain a pH between 5.5 and 7.0 to maximize the formation of HOCl and ensure effective antimicrobial action.

**Correct Answer: B. Hypochlorite ions (OCl-)**