The first ionization energy of gallium (Ga, 578.8 kJ/mol) is greater than that of aluminum (Al, 577.5 kJ/mol). Which is the best explanation for this?
a) The effective nuclear charge of gallium is greater than that of aluminum.
b) Gallium has a smaller atomic radius than aluminum.
c) The electron configuration of gallium results in greater electron-electron repulsions.
d) Gallium has a higher number of protons in its nucleus compared to aluminum.