Higher the electrostatic attraction between nucleus and valence electron, smaller will be the size of the atom\/ion. Electrostatic attraction is given by Zeff =Z−S where, Z= the number of protons in the nucleus of an atom or ion (the atomic number) and S= shielding from core electrons. In case of Al3+ and Mg2+, they are isoelectronic species and thus have same number of electrons as 1s2,2s2,2p6 isoelectric series of atoms and ions with different numbers of protons (and thus different nuclear attraction, gives) the relative ionic sizes of each atom or ion with respect to atomic number. Atomic number, ZAl=13 and ZMg=12. Thus, Al3+ has lower ionic radii than Mg2+ due to higher nuclear charge. Hence, A is true, R is true and R is the correct explanation for A.