Which of the following fcc structure contains cations in alternate tetrahedral voids?

The correct option is B $ZnS$
In $ZnS$, $S^{2-}$ (sulphide ions) are present at fee positions giving four sulphide ions per unit cell. To comply with $1:1$ stoichiometry, four $Z{n}^{2+}$ ion must be present in four alternate tetrahedral voids out of eight tetrahedral voids present. In $NaCl$, $N{a}^{+}$ ions are present in octahedral voids while in $N{a}_{2}O$, $N{a}^{+}$ ions are present in all its tetrahedral voids giving the desired $2:1$ stoichiometry. In $Ca{F}_2$, $C{a}^{2+}$ ions occupies fee positions and all the tetrahedral voids are occupied by fluoride ion.