Which of the following best represents the concept that each orbital can hold a maximum of two electrons with opposing spins?

The concept that each orbital can hold a maximum of two electrons with opposing spins is best represented by the Pauli exclusion principle. This principle states that no two electrons in an atom can have the same set of four quantum numbers, which implies that within an individual orbital, there can be a maximum of two electrons and they must have opposite spins. This characteristic allows for the differentiation of electrons within the same orbital by their spin orientation, effectively allowing them to coexist in the same space without violating the principles of quantum mechanics.

In contrast, the other principles listed pertain to different aspects of electron configuration. Hund's rule describes how electrons occupy degenerate orbitals (orbitals of the same energy) singly before pairing up, ensuring maximum total spin. The octet rule is a guideline for atoms in forming compounds, stating that atoms tend to combine in such a way that they each have eight electrons in their valence shell. The Aufbau principle outlines the order in which electrons fill orbitals, starting from the lowest energy level upwards. Each of these principles plays a distinct role in understanding electron behavior, but only the Pauli exclusion principle directly addresses the capacity and spin of electrons within an orbital.