In which of the following crystals alternate tetrahedral voids are occupied ?

In ZnS structure, sulphides occupy all the lattice points while Zn²⁺ ions are present in alternative tetrahedral voids.
$\mathrm{N}\mathrm{a}\mathrm{C}\mathrm{l}$ has FCC structure and $\mathrm{N}{\mathrm{a}}^{+}$ atom occupied all octahedral hole.
$\mathrm{C}\mathrm{a}{\mathrm{F}}_2$ has FCC structure and all T-holes will occupied by guest.
$\mathrm{N}{\mathrm{a}}_2\mathrm{O}$ has FCC sturcture and all T-holes will occupied by guest.