**Core Concept**
Residual chlorine in water treatment refers to the amount of chlorine left in the water after the disinfection process. It is essential to maintain a certain level of residual chlorine to ensure the water remains free from pathogens.

**Why the Correct Answer is Right**
The correct answer is 0.5 mg/L, as recommended by the World Health Organization (WHO). This level of residual chlorine is sufficient to maintain water quality, prevent bacterial growth, and ensure the water remains safe for consumption. The presence of 0.5 mg/L of residual chlorine also helps to prevent the growth of nitrifying bacteria, which can lead to the formation of nitrite and other toxic compounds.

**Why Each Wrong Option is Incorrect**
**Option A:** 1 mg/L is too high and can cause irritation to the eyes, skin, and respiratory system.
**Option B:** No residual chlorine would leave the water susceptible to bacterial growth and contamination.
**Option C:** 0.1 mg/L is too low and may not be sufficient to prevent the growth of certain bacteria.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the ideal level of residual chlorine may vary depending on factors such as water temperature, pH, and the presence of organic matter.

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