A **tetrahedral void** in a close-packed crystal has coordination number:

A tetrahedral void is the cavity formed at the centre of four spheres arranged at the vertices of a tetrahedron. **Coordination number is 4** (each surrounded by 4 neighbours). For a sphere to fit a tetrahedral void: $r_{\text{void}} = 0.225 \times r_{\text{sphere}}$ (the **radius ratio**). Compare with **octahedral voids** (option C): surrounded by 6 spheres, $r_{\text{void}} = 0.414 \times r_{\text{sphere}}$. Larger ions (like $\mathrm{Na^+}$ in $\mathrm{NaCl}$) fit octahedral voids; smaller ones (like $\mathrm{Zn^{2+}}$ in $\mathrm{ZnS}$) fit tetrahedral.