Analyzing Statement I: The order given is Al > Mg > Mg²⁺ > Al³⁺. However, the correct atomic radii order should be Mg > Al because Mg is in group 2 (period 3) while Al is in group 13 (period 3), and moving right across a period decreases radius. The correct order would be Mg > Al > Mg²⁺ > Al³⁺. Therefore, Statement I is FALSE.
Analyzing Statement II: Electron gain enthalpy magnitudes follow: Cl (highest, 349 kJ/mol) > Br (325) > S (200) > O (141). This is because Cl has the highest tendency to gain an electron due to its position and electronic configuration. Statement II is TRUE.
Since Statement I is false and Statement II is true, the answer is option (3).