Correct Answer - C
Elements in the same group have similar valence shell electron configuration. As we go down the group, both the nuclear charge `(Z)` and the number of inner shielding electrons `(sigma)` increase. As a result, `Z_(eff) (Z - sigma)` for the outermost electron,of each atom will be essentially the same. For example, compare `Li` and `Na`. `Al`though the number of protons has increased from `3` in `Li` to `11`in `Na`, teh latter has `10` shielding inner electrons while the former has only two. However, as the principle quantum number `n` increases, so does the average distance of a valence electron from the nucleus. A greater separation between the electron and the nucleus means a weakes attraction, so that it become increasibgly easier to remove the electron as we go from element to element down a group.
