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According to Fajan's rule of polarisation, "Small size of the cation, large size of the anion, high charge on both the ions favour covalent chracter". Greater the ionic character (i.e. lesser the covalent nature) of a binary compound, higher its lattice energy.
Among the given cations, `Mg^(2+)` has the highest charge and smallest size, followed by `Ca^(2+)` and among the anions, `S^(2-)` has the highest charge and largest size, followed by `S^(2-)` .
Therefore, the compounds CaS and MgO will have high covalent character and very close, lowest bound values of latiice energy.
Only option B) satisfies that pre-requisite. Hence the answer.
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