Assertion: Nitrobenzene is used as a solvent in Friedel-Craft's reaction.
Reason: Fusion of nitrobenzene with solid KOH gives a low yield of a mixture of o-and p-nitro phenols.

Explanation

To analyze the assertion and reason provided in the question, let's break down the statements step by step.

### Step 1: Analyze the Assertion
**Assertion:** Nitrobenzene is used as a solvent in Friedel-Crafts reaction.

- **Explanation:** Nitrobenzene is indeed used as a solvent in Friedel-Crafts reactions. This is because it can dissolve a variety of organic compounds and can withstand the reaction conditions typically used in Friedel-Crafts alkylation or acylation reactions.

### Step 2: Analyze the Reason
**Reason:** Fusion of nitrobenzene with solid KOH gives a low yield of a mixture of o-and p-nitrophenols.

- **Explanation:** When nitrobenzene is fused with solid potassium hydroxide (KOH), it undergoes a reaction that can lead to the formation of nitrophenols. However, the yield is low, and the product is a mixture of ortho (o-) and para (p-) nitrophenols. This is due to the steric and electronic factors involved in the reaction.

### Step 3: Evaluate the Relationship Between Assertion and Reason
- Both the assertion and the reason are true statements. However, the reason does not directly explain why nitrobenzene is used as a solvent in Friedel-Crafts reactions. The reason discusses a different reaction involving nitrobenzene and KOH.

### Conclusion
- **Final Answer:** The assertion is true, and the reason is also true, but the reason does not support the assertion.

---