Two electrons in the same orbital must have:
AThe same set of all four quantum numbers
BThe same $n, l, m_l, m_s$
CThe same $n, l, m_l$ but opposite spins
DDifferent orbitals but same spin
Answer & Solution
Correct answer: C. The same $n, l, m_l$ but opposite spins
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.
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