Answer:
D. Hund's rule
Explanation:
Not sure, but I would go with Hund's since it talks about filing electrons in each orbital before you can pair them up. The reason sulfur has lower ionization is because it has one set of paired electrons which makes the orbital unstable whereas phosphorus has 3 unpaired e's which means it is more stable. Thus it is easier to remove electron from sulfur hence lower ionization energy.
