Atomic radius down a group is governed by two competing factors: the addition of new electron shells and the increase in nuclear charge. Each step down a group adds a new principal energy shell, placing the outermost electrons significantly farther from the nucleus. Although the nuclear charge also rises, the added inner shells provide substantial shielding that reduces the net pull on the valence electrons. The shell-addition effect dominates, so the atomic radius increases steadily down the group. The option that shielding decreases is wrong, because additional inner shells increase shielding. The option that protons are removed is impossible. The option that electrons are removed contradicts the addition of electrons down a group. This dominance of shell number over nuclear charge is a key NCERT periodicity concept. Plausibility check: the radius increases markedly from lithium to caesium in Group 1, demonstrating that adding shells outweighs the growing nuclear charge.