**Core Concept**
Chlorine is a disinfectant commonly used in water treatment to kill pathogenic microorganisms, including Cyclops, intermediate hosts of Guinea worm disease. The effective concentration of chlorine depends on several factors, including contact time, temperature, and pH of the water.

**Why the Correct Answer is Right**
The World Health Organization (WHO) recommends a free chlorine residual of at least 0.5 mg/L for at least 30 minutes to kill Cyclops. This concentration is sufficient to inactivate the Guinea worm larvae, thereby preventing the transmission of the disease. The correct answer is based on the WHO guidelines for water disinfection.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.1 mg/L is too low to effectively kill Cyclops, as it may not provide sufficient contact time for the chlorine to inactivate the larvae.
**Option B:** 1.0 mg/L may be excessive, potentially leading to chlorination byproducts and affecting the taste and odor of the water.
**Option C:** No chlorine residual is not a viable option, as it would not provide any disinfection benefits.

**Clinical Pearl / High-Yield Fact**
Chlorine is an effective disinfectant against Cyclops, but its concentration must be carefully controlled to avoid adverse effects on human health and the environment.

**Correct Answer:** C. 0.5 mg/L