What does hund's rule state?

an orbital can contain two electrons only if the electrons have opposite spins.

an orbital can contain two electrons only if all other orbitals at that sublevel are empty.

an orbital can contain two electrons only if the electrons have different energy levels.

an orbital can contain two electrons only if all other orbitals in that sublevel contain at least one electron.

 An orbital can contain two electrons only if all other orbitals in that sublevel contain at least one electron.

Explanation: When electrons are filled in orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with the same spin direction.  then the pairing of electrons take place.

The first electron in an orbital could have up spin or down spin. All of the electrons in singly occupied orbitals have the same spin so as to maximize total spin.

Two electrons must have opposite spin is given by Pauli's exclusion principle, which states that no two electrons can have same set of quantum numbers.