## Core Concept
The Nalgonda technique is a method used for the defluoridation of water, primarily aimed at reducing the fluoride content in drinking water to prevent fluorosis. This technique involves a series of chemical treatments.

## Why the Correct Answer is Right
The Nalgonda technique involves adding lime (calcium hydroxide) and alum (aluminum sulfate) in sequence to the water. The process starts with the addition of lime to adjust the pH, which is followed by the addition of alum. Alum helps in the precipitation of fluoride as aluminum fluoride or calcium fluoride, which can then be removed. This sequence is crucial for effective defluoridation.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it does not accurately represent the sequence or the chemicals used in the Nalgonda technique.
- **Option B:** This option might seem plausible but is incorrect because the correct sequence and chemicals must match the Nalgonda technique's specifications.
- **Option D:** This option is incorrect for similar reasons as options A and B; it either misrepresents the sequence or the chemicals involved.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the Nalgonda technique is specifically used for defluoridation of water. Excessive fluoride in drinking water can lead to fluorosis, a condition characterized by discoloration and pitting of teeth and, in severe cases, skeletal fluorosis. This technique is a public health measure to prevent these conditions.

## Correct Answer: C.