Assertion : The oxidation number of fluorine in all its compounds is always -1
Reason : Fluorine is a highly reactive element .

Options

A. If both Assertion and Reason are CORRECT and Reason is the CORRECT explanation of the Assertion.
B. If both Assertion and Reason are CORRECT but Reason is not the CORRECT explanation of the Assertion.
C. If Assertion is CORRECT but Reason is INCORRECT.
D. If Assertion is INCORRECT but Reason is CORRECT.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that "the oxidation number of fluorine in all its compounds is always -1."
- This is true because fluorine is the most electronegative element, which means it tends to gain one electron to achieve a stable electronic configuration (octet).

2. **Understanding the Reason**: The reason given is "fluorine is a highly reactive element."
- This statement is also true. Fluorine's high reactivity is due to its electronegativity and small atomic size, which allows it to attract electrons strongly.

3. **Analyzing the Relationship**: We need to determine if the reason provided correctly explains the assertion.
- While both statements are true, the reason does not specifically explain why the oxidation number of fluorine is always -1. The reason focuses on fluorine's reactivity rather than its tendency to gain one electron.

4. **Conclusion**:
- The assertion is true: Fluorine has an oxidation number of -1 in all its compounds.
- The reason is true: Fluorine is highly reactive.
- However, the reason does not correctly explain the assertion. The correct explanation would involve fluorine's electronegativity and its need to gain one electron to complete its octet.

5. **Final Answer**: Both the assertion and reason are correct, but the reason is not the correct explanation for the assertion.