Assertion: Bird ringing was carried in Harike Pattan Sanctuary.
Reason: Bird ringing was helpful in bird census.

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.

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion**: The assertion states that bird ringing was carried out in Harike Pattan Sanctuary. This means that there was an activity involving the tagging or marking of birds in this specific sanctuary.

2. **Understanding the Reason**: The reason provided is that bird ringing was helpful in bird census. This implies that the activity of bird ringing contributed to counting or monitoring bird populations.

3. **Analyzing the Relationship**: We need to determine if the reason provided accurately explains the assertion. Since bird ringing allows researchers to track individual birds and gather data on their populations, it indeed aids in conducting a bird census.

4. **Evaluating the Options**:
- Option A states that both the assertion and reason are true, and the reason is the correct explanation of the assertion.
- Option B states that both are true, but the reason is not the correct explanation.
- Option C states that the assertion is true, but the reason is false.
- Option D states that both are false.

5. **Conclusion**: Since both the assertion (bird ringing was carried out in Harike Pattan Sanctuary) and the reason (bird ringing was helpful in bird census) are true and the reason correctly explains the assertion, the correct answer is Option A.