Assertion : Boron forms only covalent compounds. ,
Reason : Boron has very small size.

Answer

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

### Step 1: Understanding the Assertion
The assertion states that "Boron forms only covalent compounds."
- This is generally true. Boron, being a metalloid, tends to form covalent bonds rather than ionic bonds due to its small size and high ionization energy.

### Step 2: Understanding the Reason
The reason given is "Boron has very small size."
- Boron indeed has a small atomic radius. This small size contributes to its ability to form covalent bonds. According to Fajan's rules, smaller cations have a higher charge density, which leads to a greater tendency to polarize anions, thus favoring covalent character.

### Step 3: Linking Assertion and Reason
Now, we need to determine if the reason correctly explains the assertion.
- Since boron has a small size, it leads to the formation of covalent compounds due to its high ionization energy and the inability to lose electrons easily to form stable ionic bonds. Therefore, the reason does indeed explain the assertion.

### Conclusion
Both the assertion and the reason are true, and the reason correctly explains the assertion. Thus, the answer to the question is:
- **Both Assertion and Reason are correct, and the Reason correctly explains the Assertion.**

### Final Answer
**A. Both Assertion and Reason are correct and the Reason correctly explains the Assertion.**

---