Answer:
b. Paul's exclusion principle
Step-by-step explanation:
Let's go through the definition f each of the options and eliminate the wrong answers;
a. Aufbau's principle
This principle states that electrons fill the lower energy orbitals first before moving o higher energy orbitals. This option is wrong.
b. Paul's exclusion principle
This states that no two electrons can have the same four quantum numbers. It can also be stated that; If two electrons occupy the same orbital, they must have opposite spins. This option is correct.
c. Hund's rule
This rule states that every orbital in a sub level is singly occupied before being double occupied. This option is wrong.
d.none of the above