The addition of `CaCl_(2)` crystal to `KCl` crystal
a. Lowers the density of the `KCl` crystal
b. Raises the density of the `KCl` crystal
c. Does not affect the density of the `KCl` crystal
d. Increases the Frenkel defects of the `KCl` crystal

Explanation

To solve the problem regarding the addition of CaCl₂ crystal to KCl crystal, we need to analyze the effects of this addition on the density of the KCl crystal and the resulting structural changes.

### Step-by-Step Solution:

1. **Understanding the Initial Structure**:
- KCl is a crystalline solid composed of K⁺ and Cl⁻ ions arranged in a face-centered cubic (FCC) lattice. Each unit cell contains 4 formula units of KCl, contributing to its density.

**Hint**: Recall that the density of a crystal is determined by the mass of the ions in the unit cell and the volume of the unit cell.
...