Answer
See the answer below.
Work Step by Step
a) In Al the electron is removed from a 3p orbital, which, from Aufbau's principle, has a less negative energy level than the 3s orbital from where the electron in Mg is removed.
b) The electron removed from S has a pair in the same orbital, whereas for P it's unpaired. The repulsion between electrons explains why it's easier to remove the paired one.