Assertion : There are 4 carbon atoms in an oxaloacetic acid molecule, which joins with an acetyl group during step 1 Krebs' Citric acid cycle.
Reason : There are 6 carbon atoms in citric acid molecule.

Options

A. If both Assertion and Reason are true and Reason is correct explanation of Assertion.
B. If both Assertion and Reason are true but Reason is not a correct explanation of Assertion.
C. If Assertion is true but Reason is false.
D. If both Assertion and Reason are false.

Answer

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 there are 4 carbon atoms in an oxaloacetic acid molecule, which combines with an acetyl group during step 1 of the Krebs cycle.
- This is correct because oxaloacetic acid (C4) indeed has 4 carbon atoms, and it reacts with acetyl CoA (C2) to form citric acid.

2. **Understanding the Reason**:
- The reason states that there are 6 carbon atoms in a citric acid molecule.
- This is also correct because citric acid (C6) is formed when oxaloacetic acid (C4) combines with acetyl CoA (C2), resulting in a total of 6 carbon atoms.

3. **Analyzing the Relationship**:
- The reason explains why the assertion is true. The assertion describes the reaction that leads to the formation of citric acid, and the reason confirms the number of carbon atoms in citric acid.

4. **Conclusion**:
- Since both the assertion and the reason are true, and the reason correctly explains the assertion, the correct answer is:
- **Both assertion and reason are true, and reason is the correct explanation of assertion.**

### Final Answer:
- **Option 1**: Both assertion and reason are true, and reason is the correct explanation of assertion.