A: Fluorosis results if `F^(-)` ion concentration exceeds 10 ppm in drinking water.
R : Nalgonda techique is a cheap method for removind `F^(-)` ions from drinking water.

Answer

To solve the question, we need to analyze both the assertion (A) and the reason (R) provided in the context of fluorosis and the Nalgonda technique.

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that fluorosis occurs when the concentration of fluoride ions (F^(-)) in drinking water exceeds 10 ppm (parts per million).
- Fluorosis is a condition that affects the teeth and bones due to excessive fluoride intake, leading to discoloration and damage.

2. **Analyzing the Concentration Levels**:
- It is important to note that while fluoride is beneficial in small amounts (around 1 ppm), higher concentrations can lead to dental issues.
- Specifically, concentrations above 3 ppm can start to affect dental enamel, and levels above 7-8 ppm can lead to mottling of teeth.

3. **Conclusion on Assertion**:
- Since the assertion correctly identifies that fluorosis results from fluoride concentrations exceeding 10 ppm, we can conclude that the assertion is true.

4. **Understanding the Reason (R)**:
- The reason states that the Nalgonda technique is a cheap method for removing fluoride ions from drinking water.
- The Nalgonda technique involves using chemicals like bleaching powder, lime, and alum to precipitate and remove fluoride from water.

5. **Evaluating the Reason**:
- The Nalgonda technique is indeed a recognized and cost-effective method for reducing fluoride levels in drinking water.
- Therefore, the reason provided is also true.

6. **Determining the Relationship**:
- While both the assertion and the reason are true, the reason does not directly explain the assertion. The assertion discusses the health implications of high fluoride levels, while the reason describes a method for fluoride removal.

7. **Final Conclusion**:
- Both A and R are true, but R is not the correct explanation of A. Hence, the answer is that both statements are true, but the reason is not the correct explanation for the assertion.

### Final Answer:
- Both A and R are true, but R is not the correct explanation of A.