**Core Concept:** Residual chlorine in drinking water is important for maintaining water quality and preventing water-borne diseases. Chlorine is commonly used as a disinfectant to kill bacteria, viruses, and other microorganisms in water supply systems.

**Why the Correct Answer is Right:** The correct answer is **D**. Chlorine should be present at a residual level to ensure that the water remains safe for human consumption. The presence of residual chlorine prevents the regrowth of microorganisms after the initial disinfection process. The recommended range for residual chlorine in drinking water is between 0.2 to 0.5 mg/L (milligrams per liter).

**Why Each Wrong Option is Incorrect:**

A. **0 mg/L**: This option is incorrect because it suggests that there should be no chlorine in drinking water, which is not practical or effective in preventing water-borne diseases.

B. **High levels**: While excess chlorine might effectively kill microorganisms, it can also lead to the formation of harmful by-products like trihalomethanes, which can cause adverse health effects.

C. **Low levels (0.1 mg/L)**: A low level of residual chlorine may not be sufficient to prevent the regrowth of microorganisms, increasing the risk of water-borne infections.

**Clinical Pearl:** While chlorination is an essential method for water disinfection, it is crucial to maintain the appropriate residual chlorine levels to ensure water safety without causing harmful side effects. Monitoring and regulating chlorine levels in drinking water is a critical aspect of public health.