**Core Concept**
Fluorine is an essential element for the prevention of dental caries, but excessive intake can lead to dental and skeletal fluorosis. The safe limit of fluorine in drinking water is crucial to prevent adverse health effects while maintaining its beneficial effects on teeth.

**Why the Correct Answer is Right**
The safe limit of fluorine in drinking water is set at 1.5 mg/L by the World Health Organization (WHO) and the US Environmental Protection Agency (EPA). This limit is based on the fact that excessive intake of fluorine can lead to dental fluorosis, skeletal fluorosis, and even bone cancer. The optimal concentration of fluorine in drinking water is achieved when the fluoride ion is present in a concentration that is sufficient to prevent dental caries but not high enough to cause adverse health effects.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.5 mg/L is too low to prevent dental caries effectively.
**Option B:** 5 mg/L is excessively high and can cause dental and skeletal fluorosis.
**Option C:** 10 mg/L is much higher than the recommended safe limit and can lead to severe health problems.

**Clinical Pearl / High-Yield Fact**
The optimal concentration of fluorine in drinking water is a balance between preventing dental caries and avoiding adverse health effects. It's essential to note that the safe limit of fluorine may vary depending on the age and health status of the individual, as well as the presence of other health conditions.

**Correct Answer:** C. 1.5 mg/L is the safe limit of fluorine in drinking water.