Assertion: Nickel is purified by reaction it with `CO`.
Reason: Impurities present in nickel form volatile compounds.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion .
B. if both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "Nickel is purified by reacting it with CO."
- This is true. Nickel can be purified using the Mond process, where nickel reacts with carbon monoxide (CO) to form nickel carbonyl (Ni(CO)₄), a volatile compound.

2. **Understanding the Reason**:
- The reason states that "Impurities present in nickel form volatile compounds."
- This statement is misleading. While nickel itself forms a volatile compound (Ni(CO)₄) when reacted with CO, the impurities in nickel do not necessarily form volatile compounds. The reason is not accurate in the context of the purification process.

3. **Conclusion**:
- Since the assertion is true (nickel is purified by reaction with CO), but the reason is false (impurities do not form volatile compounds), we conclude that the correct answer is:
- Assertion is true, but reason is false.

### Final Answer:
The correct option is **C**: Assertion is true but reason is false.