A : Coordination number of both `Na^(+)` and `Cl^(-)` NaCl is 6.
R : Second coordination number of `Cl^(-)` in the NaCl unit is 12.

Options

A. If both Assertion & Reason are true and the reason is the correct explanation of the assertion then mark (1)
B. If both Assertion & Reason are true but the reason is not the correct explanation of the assertion, then mark (2)
C. If Assertion is true statement but Reason is false then mark (3)
D. If both Assertion and Reason are false statements, then mark (4)

Answer

To solve the problem, we need to analyze the coordination numbers of Na⁺ and Cl⁻ in the NaCl crystal structure and verify the statements given in the question.

### Step-by-Step Solution:

1. **Understanding Coordination Number**:
- The coordination number is defined as the number of nearest neighbor ions surrounding a particular ion in a crystal lattice.

2. **Analyzing NaCl Structure**:
- In NaCl, sodium ions (Na⁺) and chloride ions (Cl⁻) are arranged in a face-centered cubic (FCC) lattice.
- The Na⁺ ions occupy the octahedral voids in the FCC lattice formed by Cl⁻ ions.

3. **Determining the Coordination Number of Na⁺**:
- Each Na⁺ ion is surrounded by 6 Cl⁻ ions. These 6 Cl⁻ ions are located at the corners of the octahedron formed around the Na⁺ ion.
- Therefore, the coordination number of Na⁺ is 6.

4. **Determining the Coordination Number of Cl⁻**:
- Each Cl⁻ ion is surrounded by 6 Na⁺ ions in the octahedral arrangement.
- However, each Cl⁻ ion also has additional Na⁺ ions that it can interact with, located at the face centers of the cubic unit cell.
- This results in a second coordination number of 12 for Cl⁻, as it can interact with 6 Na⁺ ions in the octahedral arrangement and 6 more Na⁺ ions at the face centers.

5. **Conclusion**:
- Statement A is true: The coordination number of both Na⁺ and Cl⁻ in NaCl is 6.
- Statement R is also true: The second coordination number of Cl⁻ in the NaCl unit is 12.

### Final Answer:
Both statements A and R are true.