Assertion : `[Ni(CO)_4]` is a diamagnetic complex.
Reason : All the electrons in the complex are paired

Options

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

Answer

To solve the question, we will analyze the assertion and reason provided regarding the complex `[Ni(CO)4]`.

### Step-by-Step Solution:

1. **Identify the Complex**: The complex in question is `[Ni(CO)4]`. Here, nickel (Ni) is the central metal atom, and carbon monoxide (CO) is the ligand.

2. **Determine the Oxidation State of Nickel**: Since CO is a neutral ligand, the oxidation state of nickel in this complex is 0.

3. **Find the Electronic Configuration of Nickel**: Nickel has an atomic number of 28. Its electronic configuration is:
- \( \text{Ni: } [Ar] 3d^8 4s^2 \)

4. **Analyze the Effect of CO Ligands**: CO is a strong field ligand and causes pairing of electrons. When CO approaches nickel, it can cause the electrons in the 4s orbital to pair up with the 3d electrons.

5. **Rearranging the Electrons**: The pairing of electrons results in the following configuration:
- \( \text{Ni: } [Ar] 3d^{10} 4s^0 \)

6. **Count the Electrons in the d-Orbitals**: In the case of `[Ni(CO)4]`, all the d-orbitals are filled with paired electrons:
- \( 3d: ↑↓ ↑↓ ↑↓ ↑↓ ↑↓ \) (10 electrons, all paired)

7. **Determine Magnetic Properties**: A complex is considered diamagnetic if all electrons are paired. Since all electrons in `[Ni(CO)4]` are paired, it is indeed a diamagnetic complex.

8. **Evaluate the Assertion and Reason**:
- **Assertion**: `[Ni(CO)4]` is a diamagnetic complex. (True)
- **Reason**: All the electrons in the complex are paired. (True)

9. **Conclusion**: Since both the assertion and reason are true, and the reason correctly explains the assertion, the correct answer is that both the assertion and reason are true, and the reason is the correct explanation of the assertion.

### Final Answer:
Both the assertion and reason are true, and the reason is the correct explanation of the assertion.