**Core Concept**
The maximum permitted level of fluoride in drinking water is a critical public health measure to prevent dental fluorosis and skeletal fluorosis. The optimal fluoride concentration is crucial for maintaining oral health without causing adverse effects.

**Why the Correct Answer is Right**
The World Health Organization (WHO) recommends a maximum fluoride concentration of 1.5 mg/L (milligrams per liter) in drinking water. This level is based on the balance between preventing dental caries and minimizing the risk of dental fluorosis. Excessive fluoride intake can lead to skeletal fluorosis, a condition characterized by joint pain, bone fractures, and calcification of tendons. The kidneys play a crucial role in regulating fluoride excretion, and impaired renal function can increase the risk of fluoride toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the maximum permissible level of fluoride in drinking water is not 0.7 mg/L. This concentration would be too low to provide adequate protection against dental caries.
**Option B:** This option is incorrect because the maximum permissible level of fluoride in drinking water is not 3.5 mg/L. This concentration is higher than the recommended level and can increase the risk of dental fluorosis and skeletal fluorosis.
**Option C:** This option is incorrect because the maximum permissible level of fluoride in drinking water is not 2.0 mg/L. While this concentration is closer to the recommended level, it is still higher than the optimal range.

**Clinical Pearl / High-Yield Fact**
To remember the maximum permissible level of fluoride in drinking water, use the mnemonic "1.5 is the limit, above which is a risk to health and limb."

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