Assertion :- Midbrain, pons and medulla constitute to form brain stem.
Reason:- Medulla controls involuntary activities of our body.

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 the midbrain, pons, and medulla together form the brain stem. This is a factual statement in neuroanatomy. The brain stem is indeed composed of these three parts.

2. **Understanding the Reason**: The reason provided states that the medulla controls involuntary activities of the body. This is also true, as the medulla oblongata regulates essential functions such as breathing, heart rate, and blood pressure.

3. **Evaluating the Relationship**: While both the assertion and reason are true, the reason does not explain the assertion. The assertion is about the composition of the brain stem, while the reason focuses only on the function of the medulla. The functions of the midbrain and pons are not mentioned in the reason.

4. **Conclusion**: Since both statements are true but the reason does not serve as the correct explanation for the assertion, the correct answer is that both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

### Final Answer:
The correct option is: Both assertion and reason are true, but reason is not the correct explanation of assertion.

---