A : If a tetrad axis is passed through the unit cell of NaCl and all ions removed which are touching to tetrad axis then the formula of NaCl becomes `Na_(3)Cl_(4)`.
R : Only one `Na^(+)` is removed not the `Cl^(-)`.

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 assertion (A) and reason (R) given in the question regarding the unit cell of NaCl when a tetrad axis is passed through it.

### Step-by-Step Solution:

1. **Understanding the Structure of NaCl**:
- NaCl crystallizes in a face-centered cubic (FCC) structure.
- In one unit cell of NaCl, there are:
- 8 Na⁺ ions at the corners (each contributes 1/8 to the unit cell).
- 6 Na⁺ ions at the face centers (each contributes 1/2 to the unit cell).
- 1 Na⁺ ion at the body center (contributes 1 to the unit cell).
- Total contribution of Na⁺ ions = \(8 \times \frac{1}{8} + 6 \times \frac{1}{2} + 1 = 1 + 3 + 1 = 5\).

2. **Counting Cl⁻ Ions**:
- Similarly, for Cl⁻ ions:
- 8 Cl⁻ ions at the corners (each contributes 1/8).
- 6 Cl⁻ ions at the face centers (each contributes 1/2).
- Total contribution of Cl⁻ ions = \(8 \times \frac{1}{8} + 6 \times \frac{1}{2} = 1 + 3 = 4\).

3. **Identifying the Tetrad Axis**:
- A tetrad axis is also known as a cube diagonal.
- On the cube diagonal, there will be:
- 2 corner ions (which are Cl⁻) and 1 body-centered ion (which is Na⁺).

4. **Removing Ions Touching the Tetrad Axis**:
- When we remove the ions that touch the tetrad axis:
- We remove 2 Cl⁻ ions from the corners.
- We remove 1 Na⁺ ion from the body center.
- After removal:
- Remaining Na⁺ ions = \(5 - 1 = 4\).
- Remaining Cl⁻ ions = \(4 - 2 = 2\).

5. **Calculating the New Formula**:
- After the removal, we have:
- Na⁺ = 4
- Cl⁻ = 2
- Therefore, the new formula becomes \(Na_4Cl_2\).

6. **Conclusion**:
- The assertion (A) states that the formula becomes \(Na_3Cl_4\), which is incorrect.
- The reason (R) states that only one Na⁺ is removed and not Cl⁻, which is also incorrect since two Cl⁻ ions are removed.
- Thus, both the assertion and reason are false.

### Final Answer:
- The new formula after removing the ions touching the tetrad axis is \(Na_4Cl_2\).
- Both the assertion and reason are false.