Assertion : Magnetic susceptibillity is a pure number.
Reason : The value of magnetic susceptibility for vacuum is one.

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 "Magnetic susceptibility is a pure number."
- Magnetic susceptibility (χ) is defined as the ratio of magnetization (M) to the applied magnetic field (H), i.e., χ = M/H.
- Since it is a ratio of two quantities with the same units (both M and H are measured in terms of magnetic field), magnetic susceptibility is indeed a dimensionless quantity. Therefore, the assertion is **true**.

2. **Understanding the Reason**:
- The reason given is "The value of magnetic susceptibility for vacuum is one."
- In reality, the magnetic susceptibility of a vacuum is defined as 0, because a vacuum cannot be magnetized. Thus, the statement that the susceptibility for vacuum is one is **false**.

3. **Conclusion**:
- Since the assertion is true and the reason is false, we can conclude that the correct option is:
- **Option 3**: Assertion is true, but the reason is false.

### Final Answer:
The correct answer is that the assertion is true, but the reason is false (Option 3).

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