Which of the following factors contributes to the decrease in ionization energy within a group in the periodic table as the atomic number increases?

The increase in Atomic radius contributes to the decrease in ionization energy down a group. As you go down a group the Atomic radius increases meaning more energy levels are being added, and there are more electron shieldings. So, therefore, electron shielding blocks the pull of the nucleus from the valence electrons and it makes easier for the atom to lose its outermost electron. So the atom needs less ionization energy to remove its valence electron.