**Core Concept**
Chlorine is a disinfectant commonly used to kill waterborne pathogens in drinking water. The World Health Organization (WHO) recommends a minimum free residual chlorine concentration of at least 0.5 mg/L (ppm) to ensure the safety of drinking water.

**Why the Correct Answer is Right**
To kill cyclops, which are intermediate hosts of the Guinea worm (Dracunculus medinensis), a free residual chlorine concentration of 5 ppm is required. This is because the Guinea worm requires a specific level of chlorine to be inactivated. The WHO guidelines for water treatment recommend a chlorine dose of 2-5 ppm for effective inactivation of waterborne pathogens, including cyclops. The correct answer, 5 ppm, is the highest recommended dose, which ensures that the water is disinfected and safe for consumption.

**Why Each Wrong Option is Incorrect**
**Option A:** 3 ppm is below the recommended dose for effective inactivation of waterborne pathogens.
**Option B:** 4 ppm is slightly below the recommended dose, but not sufficient to ensure complete inactivation of cyclops.
**Option D:** 6 ppm is excessive and not necessary for killing cyclops, as the recommended dose is 5 ppm.

**Clinical Pearl / High-Yield Fact**
The WHO guidelines for water treatment recommend a chlorine dose of 2-5 ppm to ensure the safety of drinking water. This dose is effective against a wide range of waterborne pathogens, including bacteria, viruses, and protozoa.

**✓ Correct Answer: C. 5 ppm**