The correct answer is the atomic radius increase because of the higher number of occupied energy levels.
Atomic radius increases when going down a group. The valence electrons hold the higher levels because of the enhancing quantum number (n). As a consequence, the valence electrons get further away from the nucleus with the increase in n. The electron shielding inhibits these outer electrons from getting fascinated towards the nucleus. Therefore, they are held, and the resulting atomic radius is greater.