Which one of the following has the largest size ?

Atomic radii decrease across a period. Cations are smaller than their parent atoms. Among isoelectronic ions, the one with the large positive nuclear charge will have a smaller radius.

Mg and Al belongs to the same period. The size decreases along the period. So, Mg is larger in size compared to Al.

Cations are always smaller than the neutral atom, so Al³⁺ is of smallest size as it has same electrons as in Mg²⁺, but more protons to hold electrons even closer and which results in reduction of size .

Hence, overall order of size follows as: Mg>Al>Mg²⁺>Al³⁺