The ionic radii of ${\mathrm{K}}^{+},{\mathrm{Na}}^{+},{\mathrm{Al}}^{3+}$ and ${\mathrm{Mg}}^{2+}$ are in the order :

$\mathrm{Al}, \mathrm{Mg}$ and $\mathrm{Na}$ are in the third period.

Generally among the ions of the same period, higher the charge on cation smaller will be its ionic radius.

Potassium belongs to the fourth period and shall have a higher radius than the rest.

${\mathrm{Al}}^{3+}<{\mathrm{Mg}}^{2+}<{\mathrm{Na}}^{+}<{\mathrm{K}}^{+}$

Hence, the required option is B.