Assertion: The first ionization energy of Be is greater than that of B.
Reason: 2p-orbital is lower in energy than 2s-orbital.

Options

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

Explanation

To solve the question, we need to analyze the assertion and the reason provided.

### Step 1: Analyze the Assertion
The assertion states that the first ionization energy of Beryllium (Be) is greater than that of Boron (B).

- **Electronic Configuration**:
- Beryllium (Be): 1s² 2s²
- Boron (B): 1s² 2s² 2p¹

In Beryllium, both electrons in the 2s orbital are paired, while Boron has one electron in the 2p orbital, which is unpaired.

- **Ionization Energy**:
The first ionization energy is the energy required to remove the outermost electron. Since Beryllium has a completely filled 2s orbital, it is more stable and requires more energy to remove an electron compared to Boron, which has an unpaired electron in the 2p orbital.

Thus, the assertion is **True**.

### Step 2: Analyze the Reason
The reason states that the 2p orbital is lower in energy than the 2s orbital.

- **Energy Levels**:
The energy of orbitals can be determined using the n + l rule, where 'n' is the principal quantum number and 'l' is the azimuthal quantum number.
- For 2s: n = 2, l = 0 → n + l = 2 + 0 = 2
- For 2p: n = 2, l = 1 → n + l = 2 + 1 = 3

According to the n + l rule, the 2p orbital has a higher energy than the 2s orbital. Therefore, the reason provided is **False**.

### Conclusion
- The assertion is **True**: The first ionization energy of Be is greater than that of B.
- The reason is **False**: The 2p orbital is not lower in energy than the 2s orbital.

### Final Answer
The correct option is that the assertion is true but the reason is false (Assertion True, Reason False).