## Core Concept
Defluoridation of water refers to the process of removing excess fluoride from water to prevent fluorosis, a condition caused by excessive intake of fluoride. This process is crucial in areas where the fluoride levels in water are higher than the recommended standards. Various methods can be employed for defluoridation, including chemical and physical treatments.

## Why the Correct Answer is Right
The correct answer, **Nalco Process**, involves the use of alum (aluminum sulfate) or other aluminum salts to remove fluoride from water. In this process, lime is added to adjust the pH, and then alum is added to precipitate fluoride as aluminum fluoride or to form complexes that reduce free fluoride ions in the water. This method is effective in reducing fluoride levels to acceptable standards.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not specify a known defluoridation method.
- **Option B:** This option is incorrect as it also does not represent a recognized technique for defluoridation of water.
- **Option C:** This option might represent another water treatment method but is not the correct answer for defluoridation.

## Clinical Pearl / High-Yield Fact
A key point to remember is that excessive fluoride consumption can lead to dental and skeletal fluorosis. The World Health Organization (WHO) recommends a maximum limit of 1.5 mg/L of fluoride in drinking water. Defluoridation methods like the Nalco Process are critical in regions with high fluoride levels in groundwater.

## Correct Answer Line
**Correct Answer: D. Nalco Process**