**Core Concept**
The Nalgonda technique is a method of defluoridation of water, which involves the addition of a chemical agent to reduce the fluoride content. This technique is essential in areas where excessive fluoride intake can lead to health issues, such as fluorosis.

**Why the Correct Answer is Right**
The correct answer involves the addition of lime (calcium hydroxide) in the Nalgonda technique, followed by the addition of alum (aluminum sulfate). The lime raises the pH of the water, making it less soluble for the fluoride ions, which then precipitate out as calcium fluoride. The alum, added subsequently, helps to coagulate the precipitated calcium fluoride, making it easier to remove from the water.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not specify the correct chemical agents used in the Nalgonda technique.
* **Option B:** This option is incorrect because it is not the correct sequence of chemical addition in the Nalgonda technique.
* **Option C:** This option is incorrect because it does not accurately represent the chemical agents used in the Nalgonda technique.

**Clinical Pearl / High-Yield Fact**
The Nalgonda technique is a low-cost and effective method of defluoridation, which can be implemented in areas where access to more advanced water treatment technologies is limited. It is essential to follow the correct sequence of chemical addition to ensure the effectiveness of the technique.

**Correct Answer:** C. Lime followed by alum.