Which of the fallowing has maximum covalent character with S^2- ?

Number of shells increases size increases down the group in alkali metals.

cation size$\infty \frac{1}{\mathrm{polarizing} \mathrm{power}}\infty \mathrm{ionic} \mathrm{charecter}\infty \mathrm{melting} \mathrm{point} \infty \frac{1}{\mathrm{covalent} \mathrm{character}}$, hence Mg⁺² ion has maximum polarizing power, so it maximum covalent character.