Okay, let's tackle this question. The question is about the residual chlorine required in a normal water supply system to disinfect water-borne pathogens. Hmm, I remember that chlorine is commonly used for water disinfection. The key here is residual chlorine, which means the amount left in the water after treatment to continue killing pathogens as the water travels through the distribution system.

First, I need to recall the standard levels. From what I've studied, the World Health Organization (WHO) has guidelines for drinking water. The initial chlorine concentration might be higher, but residual chlorine is crucial. I think the acceptable range for free chlorine residual is between 0.2 to 0.5 mg/L. This ensures that any pathogens introduced in the distribution system are killed. If it's too low, there's a risk of contamination; too high, it can cause taste issues or health concerns.

Looking at the options, even though they're not provided here, common distractors might include values like 0.05 mg/L (too low), 1.0 mg/L (too high), or maybe 0.1 mg/L. The correct answer would be around 0.2-0.5 mg/L. So if the options had these, the correct one would be in that range.

Why the other options are wrong: For example, if an option was 0.05 mg/L, that's below the recommended level and wouldn't effectively disinfect. If another option was 1.0 mg/L, that's above the safe limit and could be harmful. The key is maintaining a balance.

Clinical pearl: Remember that residual chlorine is necessary to maintain water safety throughout the distribution network. The WHO recommends a free chlorine residual of 0.2–0.5 mg/L at the point of use. Also, total chlorine includes both free and combined forms, but free chlorine is more effective for disinfection. This is a high-yield fact for exams because it's a common question on water treatment standards.

**Core Concept**
Residual chlorine in water supply systems refers to the amount of free chlorine remaining after disinfection to inactivate pathogens during water storage and distribution. The World Health Organization (WHO) recommends **0.2–0.5 mg/L** of free chlorine residual for safe drinking water.

**Why the Correct Answer is Right**
Free chlorine (hypochlorous acid and hypochlorite ions) disrupts microbial cell membranes and oxidizes essential cellular components, killing bacteria, viruses, and protozoa. A residual concentration of **0.2–0.5 mg/L** ensures continued disinfection as water travels through pipelines, preventing recontamination. This range balances efficacy with safety, avoiding toxic byproducts from excessive chlorination.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.05 mg/L is below the WHO minimum and fails to inactivate pathogens effectively.
**Option B:** 1.0 mg/L exceeds safe limits, risking health effects like respiratory irritation or carcinogenic byproducts.
**Option C:** 0.1 mg/L is insufficient to maintain disinfectant action in distribution systems.

**Clinical Pearl / High-Yield Fact**
Remember the WHO guideline: **0.2–0.5 mg/L free chlorine residual** at the point of use. Exams often test this range versus total chlorine (0.5–

✓ Correct Answer: A. 0.5mg/L