Assertion (A) The highest oxidation state of osmium is +8.
Reason (R ) Osmium is a 5d-block element.

Options

A. Both assertion and reason are true, and reason is the correct explanation of the assertion.
B. Both assertion and reason are true but reason is not the correct explanation of assertion.
C. Assertion is not true but reason is true.
D. Both assertion and reason are false.

Explanation

To solve the question regarding the assertion and reason about osmium, we will analyze both statements step by step.

### Step 1: Analyze the Assertion
**Assertion (A)**: The highest oxidation state of osmium is +8.

- Osmium (Os) is a transition metal located in the 5d block of the periodic table.
- The common oxidation states of osmium include +2, +3, +4, +6, and +8.
- The +8 oxidation state is achieved when osmium donates all of its d and s electrons.
- The electronic configuration of osmium is [Xe] 4f14 5d6 6s2.
- In the +8 oxidation state, osmium loses all six 5d electrons and both 6s electrons, resulting in a total of eight electrons lost.

**Conclusion for Step 1**: The assertion is true.

### Step 2: Analyze the Reason
**Reason (R)**: Osmium is a 5d-block element.

- Osmium is indeed a 5d-block element, which means it is part of the transition metals where the d-orbitals are being filled.
- The transition metals can exhibit a variety of oxidation states due to the involvement of d electrons in bonding.
- However, being a 5d-block element alone does not explain why osmium can achieve a +8 oxidation state.

**Conclusion for Step 2**: The reason is true, but it does not provide a complete explanation for the assertion.

### Step 3: Determine the Relationship
- Both the assertion and the reason are true.
- However, the reason does not adequately explain the assertion. The ability of osmium to reach the +8 oxidation state is more closely related to factors such as the effective nuclear charge and the inert pair effect, rather than simply its classification as a 5d-block element.

### Final Conclusion
- The correct answer is that both the assertion and reason are true, but the reason is not the correct explanation for the assertion.

### Answer:
Both Assertion (A) and Reason (R) are true, but R is not the correct explanation of A.