In each of the following questions a Statement of Assertion (A ) is given followed by a corresponding statement of Reason (R ) Just below it . Of the statement mark the correct answer as
Assertion . The highest oxidation state of Os is + 8.
Reason . Osmium is a 5d- block element .

Explanation

To solve the question regarding the Assertion (A) and Reason (R) statements about osmium, we can break it down step by step.

### Step 1: Analyze the Assertion
The assertion states that "The highest oxidation state of Os is +8."
- To verify this, we need to consider the electronic configuration of osmium (Os), which has an atomic number of 76. The electronic configuration is:
\[ \text{Xe} \, 4f^{14} \, 5d^6 \, 6s^2 \]
- In this configuration, osmium has a total of 8 valence electrons (6 from 5d and 2 from 6s). The highest oxidation state is determined by the maximum number of electrons that can be lost, which is indeed +8.

### Step 2: Analyze the Reason
The reason states that "Osmium is a 5d-block element."
- This is true, as osmium is located in the 5d block of the periodic table. However, the reason provided does not directly explain why the highest oxidation state is +8. The oxidation state is determined by the number of valence electrons available for bonding and not solely by the block in which the element is found.

### Step 3: Conclusion
- Both the assertion and the reason are true statements. However, the reason does not correctly explain the assertion. Therefore, the correct answer is that the assertion is true, but the reason is not a correct explanation of the assertion.

### Final Answer
The correct answer is:
**Assertion is true, Reason is true, but Reason is not the correct explanation for Assertion.**