The correct option is C Pauli's exclusion principle
According to Pauli's exclusion principle, two electrons cannot have the same quantum numbers, in a given atom. Therefore, when two identical atoms combine to form a diatomic molecule, each atomic orbital (i.e. energy level) splits up into two different molecular orbitals (having different energies).
This allows the electrons in the first atomic orbitals to occupy a new orbital structure, but none of the molecular orbitals have the same energy.
Hence, (B) is the correct answer.