Final answer:
The second ionization energy of Al is greater than that of Mg, because removing a second electron from Al disrupts a stable noble gas configuration, whereas Mg still has an electron in the 3s subshell.
Step-by-step explanation:
The second ionization energy refers to the energy required to remove a second electron from an atom. For aluminum (Al), the second ionization energy is greater than that of magnesium (Mg). This is because, after the first electron is removed from Al, it achieves a noble gas configuration (Ne structure), which is a stable and low-energy state. Removing an additional electron from this stable configuration requires a lot of energy.
On the other hand, magnesium (Mg) after losing one electron still has an electron in the 3s subshell. So, its second ionization involves removing an electron from a filled s subshell which comparatively requires less energy than removing an electron from an already stable noble gas configuration as in Al.
Therefore, based on this understanding, the correct option is (a) It is greater than that of Mg.