**Core Concept**
The residual chlorine in a normal supply water system is a critical parameter to ensure the water is safe for human consumption. The residual chlorine level acts as a disinfectant, killing bacteria and other microorganisms that can cause waterborne diseases. The World Health Organization (WHO) recommends a minimum residual chlorine level of 0.5 mg/L in drinking water.

**Why the Correct Answer is Right**
The residual chlorine level in a normal supply water system is typically maintained between 0.5 and 1.0 mg/L. This range is sufficient to inactivate 99.99% of bacteria, viruses, and other pathogens that may be present in the water. The chlorine is added at the water treatment plant and is usually measured at the point of entry into the distribution system. The residual chlorine level is then monitored regularly to ensure it remains within the acceptable range.

**Why Each Wrong Option is Incorrect**
**Option A:** 1.5 mg/L is too high and can cause irritation to the eyes, skin, and respiratory tract in some individuals.
**Option B:** 0.1 mg/L is too low and may not provide adequate disinfection, increasing the risk of waterborne diseases.
**Option C:** 2.0 mg/L is excessively high and can lead to the formation of harmful byproducts, such as trihalomethanes (THMs).

**Clinical Pearl / High-Yield Fact**
It's essential to note that the residual chlorine level can fluctuate depending on factors such as water temperature, pH, and flow rate. Regular monitoring of residual chlorine levels is crucial to ensure the water supply remains safe for consumption.

**Correct Answer: D. 0.5 - 1.0 mg/L**