**Core Concept**
Fluoride is an essential element in preventing dental caries, and its optimal concentration in drinking water is crucial for public health. The recommended level of fluoride in drinking water is carefully balanced to prevent both tooth decay and fluorosis, a condition characterized by discoloration and pitting of tooth enamel.

**Why the Correct Answer is Right**
The World Health Organization (WHO) recommends a fluoride concentration of 0.5-0.8 mg/L in drinking water to prevent dental caries effectively while minimizing the risk of fluorosis. This optimal range is achieved by considering the fluoride intake from various sources, including drinking water, toothpaste, and food. The optimal concentration is also influenced by the individual's age, diet, and oral health status.

**Why Each Wrong Option is Incorrect**
**Option A:** 0.2-0.5 mg/L is too low to provide adequate protection against dental caries, as it may not be sufficient to achieve the desired level of fluoride in saliva and tooth enamel.

**Option C:** 0.8-1.2 mg/L is higher than the recommended range, increasing the risk of fluorosis, particularly in children and adolescents whose teeth are more susceptible to the effects of excessive fluoride.

**Option D:** 1.2-2.0 mg/L is excessively high, potentially leading to severe fluorosis and other health issues, such as skeletal fluorosis, which can cause joint pain and bone damage.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the optimal fluoride level in drinking water may vary depending on the local climate, geology, and water quality. Therefore, it is crucial to monitor and adjust the fluoride concentration in drinking water according to the specific needs of the community.

**✓ Correct Answer: B. 0.5-0.8 mg/L**