Assertion : Oxygen is a diatomic element.
Reason : The elements each of whose molecules contain two atoms is called diatomic element.

Options

A. If both assertion and reason are true and reason is the correct explanation
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

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 "Oxygen is a diatomic element."
- We know that oxygen exists in nature primarily as O2, which means it consists of two oxygen atoms bonded together.

2. **Understanding the Reason**:
- The reason given is "The elements each of whose molecules contain two atoms is called a diatomic element."
- This definition aligns with what we know about diatomic elements. A diatomic element is indeed one that consists of molecules containing two atoms of the same or different elements.

3. **Evaluating the Assertion**:
- Since oxygen exists as O2, it confirms that the assertion is true.

4. **Evaluating the Reason**:
- The reason correctly defines what a diatomic element is, which supports the assertion.

5. **Conclusion**:
- Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, we conclude that both the assertion and reason are correct, and the reason is the correct explanation for the assertion.

### Final Answer:
Both assertion and reason are true, and the reason is the correct explanation for the assertion.