Which of the following correctly describes the electron configuration rule?

The chosen answer correctly describes the electron configuration rule known as Hund's rule. This rule states that when electrons occupy degenerate orbitals (orbitals of the same energy), they will first fill each orbital singly, with parallel spins, before any orbital gets filled with a second electron. This approach minimizes repulsion between electrons and is energetically favorable.

In addition to this fundamental aspect of electron filling, it's important to understand that the other possible descriptions either misinterpret the rules of electron configuration or inaccurately represent the order of filling orbitals. For example, pairing electrons in an orbital before all degenerate orbitals are singly occupied contradicts Hund's rule. Additionally, while the spins of paired electrons must be opposite (not identical), simply stating that they must have identical spins is misleading with respect to how electrons typically fill their orbitals. Lastly, stating that all orbitals must be filled before beginning a new shell generalizes the rule incorrectly, as often one must fill orbitals within a shell to adhere to the Aufbau principle.

Thus, the selection of the second answer aligns perfectly with the well-established rules of electron configurations used in chemistry.