Which of the following states that no more than two electrons can occupy an atomic orbital and that two electrons in the same orbital must have opposite spins?
a. hund's rule
b. dalton's theory
c. the aufbau principle
d. the pauli exclusion principle

The correct answer is D. The statement that no more than two electrons can occupy an atomic orbital and that two electrons in the same orbital must have opposite spins is from the Pauli Exclusion Principle. If one is in the +1/2 up-spin the other should be in the -1/2 down-spin. From this principle, no two electrons can have the same quantum numbers.