For which of the following cases, answer is `4`

Options

A. Coordination number of `Zn^(2+)` in `Z` inc blende
B. Number of body diagonal planes in a cube
C. Formula units in rock salt structure
D. Formula units in `CsCl` structure

Explanation

To solve the question "For which of the following cases, the answer is `4`", we need to analyze each option provided in the context of solid-state chemistry.

### Step-by-Step Solution

1. **Coordination Number of Zn²⁺ in Zinc Blend (ZnS)**:
- In the zinc blend structure, the sulfide ions (S²⁻) form a face-centered cubic (FCC) lattice.
- The zinc ions (Zn²⁺) occupy half of the tetrahedral voids in this lattice.
- In an FCC lattice, there are a total of 8 tetrahedral voids.
- Since Zn²⁺ occupies half of these, it occupies 4 tetrahedral voids.
- Therefore, the coordination number of Zn²⁺ is **4**.

2. **Number of Body Diagonal Planes in a Cube**:
- A cube has 6 body diagonal planes.
- Therefore, this does not give us the answer of 4.

3. **Formula Units in a Rock Salt Structure (NaCl)**:
- The rock salt structure is also an FCC type structure where Cl⁻ ions form the FCC lattice.
- Na⁺ ions occupy all the octahedral voids in this structure.
- In an FCC lattice, the number of formula units is given by the number of lattice points in the unit cell.
- For NaCl, there are 4 formula units per unit cell.
- Therefore, this gives us the answer of **4**.

4. **Formula Units in CsCl**:
- CsCl forms a simple cubic structure.
- In a simple cubic structure, there is only 1 formula unit per unit cell.
- Therefore, this does not give us the answer of 4.

### Conclusion
From the analysis:
- The coordination number of Zn²⁺ in Zinc blend is **4**.
- The number of formula units in a rock salt structure (NaCl) is also **4**.

Thus, the cases for which the answer is `4` are:
1. Coordination number of Zn²⁺ in Zinc blend.
2. Formula units in a rock salt structure (NaCl).