The correct answer is Mg^(2+). Explanation & Key Takeaways: • The energy needed to remove one or more electrons from a neutral atom to form a positively charged ion is a physical property that influences the chemical behavior of the atom. • By definition, the first ionization energy of an element is the energy needed to remove the outermost, or highest energy, electron from a neutral atom in gaseous state. • The ionization energy is different for electrons of different atomic or molecular orbitals. • The below-given figure depicts the various ionization energies of the Magnesium atom. • It is visible that the second ionization energy of Mg is larger than the first because it always takes more energy to remove an electron from a positively charged ion than from a neutral atom. • The third ionization energy of magnesium is enormous, however, because the Mg2+ ion has a filled-shell electron configuration. • Hence the correct answer is Mg2+ as it has maximum ionization energy.