Which one of the following is the correct order of increase in size?

The atomic radius increases decrease across the period and increase down the group.
More the positive charge, the smaller the ionic radius. Negative means adding an electron. These electrons go into the outermost shells. Also, when an atom loses an electron, it clings ever more tightly to the ones it has left, further reducing the ionic radius.

 Hence, the correct option is ${\mathrm{Na}}^{+}<\mathrm{Al}<{\mathrm{Mg}}_{\mathrm{g}}<{\mathrm{O}}^{2-}$