**Core Concept**
The World Health Organization (WHO) has established guidelines for drinking water quality to ensure public health safety. These guidelines cover various parameters such as microbial, chemical, and physical aspects of water quality.

**Why the Correct Answer is Right**
The WHO guidelines for drinking water quality recommend a maximum permissible limit of 0.3 mg/L for Fluoride (F-) to prevent dental and skeletal fluorosis. The optimal level of fluoride in drinking water is crucial to prevent tooth decay while minimizing the risk of adverse health effects. The guideline value is based on the understanding that excessive fluoride intake can lead to dental and skeletal fluorosis, particularly in children and individuals with impaired renal function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect as the WHO guidelines do not specify a maximum permissible limit for Chloride (Cl-) in drinking water.
**Option B:** This option is incorrect as the WHO guidelines do not recommend a maximum permissible limit for Calcium (Ca2+) in drinking water.
**Option C:** This option is incorrect as the WHO guidelines do not specify a maximum permissible limit for Sulfate (SO42-) in drinking water.

**Clinical Pearl / High-Yield Fact**
It's essential to note that the optimal level of fluoride in drinking water varies depending on the geographical location and climate. In areas where the fluoride concentration in drinking water is naturally high, it may be necessary to adjust the recommended level to prevent adverse health effects.

**Correct Answer: D. 0.3 mg/L**