## Core Concept
The process of defluoridation of water involves removing excess fluoride from water to make it safe for drinking. This is crucial because excessive fluoride consumption can lead to **fluorosis**, a condition characterized by discoloration and pitting of teeth, and in severe cases, skeletal fluorosis. Defluoridation methods are essential in regions where the groundwater has high fluoride levels.

## Why the Correct Answer is Right
The correct answer, **Nalco Process**, refers to a method used for the defluoridation of water. This process involves the use of **aluminium salts** (like aluminium sulphate) or other chemicals that react with fluoride in water to form insoluble compounds, which are then removed. The Nalco process is specifically designed to reduce fluoride levels in water to acceptable limits, making it safe for consumption.

## 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 does not represent a recognized technique for defluoridation.
- **Option D:** This option is incorrect because it does not correspond to a valid defluoridation process.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the **permissible limit of fluoride in drinking water** is generally considered to be 1.5 mg/L (or ppm) by the World Health Organization (WHO). Excessive fluoride can lead to **dental and skeletal fluorosis**. Defluoridation processes like the Nalco Process are critical in areas where natural water sources exceed these levels.

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