**Core Concept**
The permissible level of fluoride in drinking water is a critical public health concern due to its potential to cause dental fluorosis and skeletal fluorosis. The optimal concentration of fluoride in drinking water is essential to prevent these conditions while providing the benefits of fluoride in preventing dental caries.

**Why the Correct Answer is Right**
The permissible level of fluoride in drinking water is typically set between 0.7 and 1.2 milligrams per liter (mg/L) by the World Health Organization (WHO) and the U.S. Environmental Protection Agency (EPA). This range allows for the prevention of dental caries while minimizing the risk of dental and skeletal fluorosis. The optimal concentration of fluoride in drinking water is achieved by adjusting the fluoride content based on the climate, water temperature, and population demographics.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because a fluoride level of 0.5 mg/L is below the recommended concentration for preventing dental caries. While it may not cause immediate harm, it is not sufficient to provide the optimal benefits of fluoride in preventing dental caries.
**Option B:** This option is incorrect because a fluoride level of 2.0 mg/L is above the recommended concentration for preventing dental caries and may cause skeletal fluorosis. High concentrations of fluoride can lead to an increased risk of dental and skeletal fluorosis.
**Option C:** This option is incorrect because a fluoride level of 1.5 mg/L is above the maximum recommended concentration for preventing dental caries and may cause skeletal fluorosis. While it may provide some benefits in preventing dental caries, it increases the risk of dental and skeletal fluorosis.
**Option D:** This option is incorrect because a fluoride level of 0.3 mg/L is below the recommended concentration for preventing dental caries. While it may not cause immediate harm, it is not sufficient to provide the optimal benefits of fluoride in preventing dental caries.

**Clinical Pearl / High-Yield Fact**
The optimal concentration of fluoride in drinking water is 0.7 to 1.2 mg/L, and the risk of dental and skeletal fluorosis increases with concentrations above 1.2 mg/L. Regular monitoring of fluoride levels in drinking water is essential to prevent these conditions.

**Correct Answer:** C. 1.5 mg/L.