**Core Concept**
Fluoride content in drinking water is a critical factor in preventing dental caries while minimizing the risk of dental fluorosis and other adverse effects. The optimal fluoride concentration in drinking water is a delicate balance between caries prevention and potential toxicity.

**Why the Correct Answer is Right**
The recommended optimal fluoride concentration in drinking water is between 0.5 and 1.5 milligrams per liter (mg/L). This range allows for the prevention of dental caries while minimizing the risk of dental fluorosis, a condition characterized by mottling of the teeth. The mechanism of action of fluoride involves the inhibition of acid production by bacteria in the mouth, thereby reducing the risk of tooth decay. Additionally, fluoride ions can also promote the remineralization of tooth enamel, making teeth more resistant to decay.

**Why Each Wrong Option is Incorrect**
**Option A:** 4 mg/L is toxic and can cause skeletal fluorosis, a condition characterized by joint pain and calcification of ligaments.

**Clinical Pearl / High-Yield Fact**
The optimal fluoride concentration in drinking water is a balance between 0.5 and 1.5 mg/L, as higher concentrations can lead to dental fluorosis and skeletal fluorosis.

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