Assertion: In the body centred cubic structure of CsCl the arrangement of `Cl^(-)` ions is primitive
Reason: In CsCl, the `Cs^(+)` ion remains at the body centred position and `Cl^(-)` ions at the corners.

Options

A. If both assertion and reason are correct and reason is correct explanation for assertion
B. If both assertion and reason are correct but reason is not correct explanation for assertion
C. If assertion is correct but reason is incorrect
D. If both assertion and reason are incorrect

Answer

To solve the question, we need to analyze the assertion and the reason provided regarding the structure of CsCl in a body-centered cubic (BCC) arrangement.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that in the body-centered cubic structure of CsCl, the arrangement of Cl⁻ ions is primitive.
- A primitive unit cell is defined as one that contains lattice points only at the corners of the cell.

2. **Understanding the Reason**:
- The reason states that in CsCl, the Cs⁺ ion is located at the body center of the cube, while the Cl⁻ ions are positioned at the corners.
- In a BCC structure, there is one atom at the body center and eight atoms at the corners of the cube.

3. **Analyzing the CsCl Structure**:
- In the CsCl structure, the Cs⁺ ion occupies the body center of the cubic lattice.
- The Cl⁻ ions are located at the eight corners of the cube.

4. **Determining the Type of Unit Cell**:
- Since the Cl⁻ ions are only at the corners of the unit cell, this satisfies the condition for a primitive unit cell.
- Therefore, the Cl⁻ ions indeed form a primitive unit cell because they are present only at the corners.

5. **Conclusion**:
- Both the assertion and the reason are correct.
- The reason provided correctly explains the assertion, as the arrangement of Cl⁻ ions at the corners of the unit cell leads to the conclusion that it is a primitive arrangement.

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