Ionic radii

The ratio $\frac{r_{cation}}{r_{anion}}$ is highest when the cation is the largest and the anion is the smallest.
Cation sizes: $Li^+ < Na^+ < Cs^+$, so $Cs^+$ is the largest cation.
Anion sizes: $F^- < I^-$, so $F^-$ is the smallest anion.
The combination of the largest cation with the smallest anion is CsF.
Hence CsF has the highest cation to anion size ratio.

All four ions have 18 electrons (isoelectronic), but different nuclear charges: $S^{2-}$ (Z = 16), $Cl^-$ (Z = 17), $K^+$ (Z = 19), $Ca^{2+}$ (Z = 20).
For isoelectronic species, the greater the nuclear charge, the more strongly the same number of electrons is pulled in and the smaller the ion.
So the radius decreases as Z increases: $S^{2-}$ (184 pm) > $Cl^-$ (181 pm) > $K^+$ (138 pm) > $Ca^{2+}$ (100 pm).
Decreasing order of ionic radii: $S^{2-} > Cl^- > K^+ > Ca^{2+}$