Assertion : Poliomyelitis is a highly infectious viral disease of infants and children.
Reason : Its causal agent is a very small RNA containing virus, Polio virus.

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.

Explanation

**Step-by-Step Solution:**

1. **Understanding the Assertion:**
- The assertion states that "Poliomyelitis is a highly infectious viral disease of infants and children." This is true as poliomyelitis primarily affects young children and is known to be highly contagious.

2. **Understanding the Reason:**
- The reason given is "Its causal agent is a very small RNA containing virus, Polio virus." This statement is also true. The poliovirus is indeed a small RNA virus responsible for causing poliomyelitis.

3. **Analyzing the Relationship Between Assertion and Reason:**
- While both the assertion and the reason are correct, we need to evaluate if the reason provides a correct explanation for the assertion. The reason explains what causes poliomyelitis but does not explain why it is highly infectious or specifically why it affects infants and children more than adults.

4. **Conclusion:**
- Since both statements are correct but the reason does not adequately explain the assertion, we can conclude that the assertion is true, the reason is true, but the reason is not a correct explanation of the assertion.

5. **Final Answer:**
- The correct option is that both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

---