Which of the following ions, has polarizing power close to that of ${\mathrm{Mg}}^{+2}$

Lithium and magnesium show diagonal relationship arise because of similar sizes.  Ionic radii of Li+=76 pm  and ionic radii of ${\mathrm{Mg}}^{+2}=72 \mathrm{pm}$

Both  ${\mathrm{Li}}^{+},{\mathrm{Mg}}^{+2}$  have high polarizing power (ionic potential).