Ionization energy decreases down the group, from top to bottom, due to:

Explanation

Ionization energy is the energy required to remove an electron from the outermost shell of the atom. The factors that affect activation energy are:An increase in proton number, as more protons cause a greater electrostatic force of attraction between the nucleus and the outermost electrons, requiring more energy to overcome. A decrease in atomic size, as the force of attraction is inversely proportional to distance so, the closer the atoms are to the nucleus, the greater would be the force of attraction, which requires greater energy to overcome.The shielding effect describes the reduction in effective nuclear force of attraction between the nucleus and outer electrons due to the repulsion exerted by the inner shells of the atom. As we go down the group, the proton number increases, which would theoretically cause ionization energy to increase, but as we go down the group, the number of shells of each atom increases, successively, resulting in a greater shielding effect which reduces the effective force of attraction between the nucleus and the valence electrons. Hence, ionization energy decreases due to an increase in shielding effect down the group, i.e. option B.Option A does not have a direct impact on ionization energy, albeit atomic mass is accompanied by atomic number, closely in most atoms.Option C is a correct observation down the group, but the ionization energy still decreases even though the atomic number increases.Option D is an incorrect statement as atomic size increases down the group.