In a Crystal of CsCl, the nearest neighbours of each Cs ion are:

eight chloride ions

The answer is 4. 8 chloride ions.

CsCl has a body-centered cubic (bcc) structure. In this structure, each Cs ion is surrounded by 8 Cl ions, and each Cl ion is surrounded by 8 Cs ions.

Here is a diagram of the CsCl crystal structure:

The blue spheres represent Cs ions and the green spheres represent Cl ions. As you can see, each Cs ion is surrounded by 8 Cl ions, and each Cl ion is surrounded by 8 Cs ions.

The coordination number of an ion is the number of ions of the opposite charge that surround it. In the case of CsCl, the coordination number of both the Cs ion and the Cl ion is 8.