Two electrons in the same orbital must have:

**Pauli's exclusion principle:** no two electrons in an atom can have all four quantum numbers identical. Electrons sharing one orbital agree on $n, l, m_l$ (they have to, that's what an orbital is), so they must differ in the fourth: spin. One is $+\tfrac{1}{2}$ and the other $-\tfrac{1}{2}$. Direct consequence: each orbital holds at most 2 electrons, with paired antiparallel spins. Option A directly contradicts Pauli. Option C would mean two electrons identical in every property.