Assertion : ZnS has a body centred cubic arrangement.
Reason : In `ZnS, Zn^(2+)`, ions occupy alternate tetrahedral sites while `S^(2-)` ions form cubic close packed structure.

Explanation

To solve the question, we need to analyze the assertion and the reason provided about Zinc Sulfide (ZnS).

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that "ZnS has a body-centered cubic arrangement."
- This is incorrect. ZnS actually crystallizes in a cubic structure known as the zinc blende structure, which is a face-centered cubic (FCC) arrangement, not body-centered cubic (BCC).

2. **Understanding the Reason**:
- The reason states that "In ZnS, Zn²⁺ ions occupy alternate tetrahedral sites while S²⁻ ions form a cubic close-packed structure."
- This statement is correct. In the zinc blende structure, the S²⁻ ions form a face-centered cubic (FCC) lattice, and the Zn²⁺ ions occupy half of the tetrahedral voids.

3. **Conclusion**:
- Since the assertion is false and the reason is true, we can conclude that the overall statement is incorrect. The correct relationship is that the reason supports the structure of ZnS, but the assertion about its arrangement being BCC is wrong.

4. **Final Answer**:
- The correct option is C: Assertion is false, but Reason is true.