In which of the following compound size of cation to anion ratio is minimum?

The correct option is B LiI 

The given anions are ${\mathrm{F}}^{-}$ and ${\mathrm{I}}^{-}$ and the size (R) order is ${\mathrm{R}}_{\mathrm{F}}<{\mathrm{R}}_{\mathrm{I}}$ The given cations are ${\mathrm{Li}}^{+}$ and ${\mathrm{Cs}}^{+}$and the size (R) order is ${\mathrm{R}}_{\mathrm{Li}}<{\mathrm{R}}_{\mathrm{Cs}}$ For the cation to anion ratio to be minimum, the compound must have a large anion, and a small cation. Here, ${\mathrm{I}}^{-}$ is the largest anion and ${\mathrm{Li}}^{+}$ is the smallest cation. Therefore, LiI has the minimum cation to anion size ratio.