Which of the following species will have the largest and the smallest size `Mg, Mg^(2+), Al, Al^(3+)`?

Explanation

Atomic radii decrease across a period. Cations are smaller than their parent atoms. Among isoelectronic species, the one with the larger positive nuclear charge will have a smaller radius.
Hence the largest species is Mg, the smallest one is `Al^(3+)`.