What would happen if the number of protons were to change in an atom

What does hund's rule state? a. an orbital can contain two electrons only if the electrons have opposite spins. b. an orbital can contain two electrons only if all other orbitals at that sublevel are empty. c. an orbital can contain two electrons only if the electrons have different energy levels. d. an orbital can contain two electrons only if all other orbitals in that sublevel contain at least one electron.