Answer :
From the options, it is better for us to try to identify which ones are not correct.
A. Aufbau's principle says that, in an ground state atom, the electrons fill the atomic orbitals with lower energy first. However, it doesn't say anything about comparing one kind of atom with other, like P and S. Actually, if we just take Aufbau's principle into consideration, we would think that phosphorus has the lower ionization energy. So, this is no the correct alternative.
B. Conservation of energy is important when we are transforming or transfering energy, but here we are just talking about one state, so there is nothing regarding conservation of energy here. So, this is no the correct alternative.
C. Hund's rule says about how suborbitals are filled, that is, first single fill each wwith same spin orientation than starting double filling. This doesn't help us compare P and S ionization energies.
The actual reason is that S has p³ filled, that is, it is half-filled. Half filled orbitals are more stable than orbitals that are filled with more than half electrons, which is the case for P, that has p⁴. This makes it easier to remove the 4th electron from P than the third electron from S.
Thus, the correct alternative is D. None of these.
