Final answer:
Hund's rule states that each orbital in a sublevel is first occupied by one electron with parallel spins before any pairing occurs. Therefore, the correct answer is a) One electron.
Step-by-step explanation:
Hund's rule dictates how electrons are distributed among orbitals within an atom. Specifically, Hund's rule states that orbitals of equal energy are each occupied by one electron before any orbital is occupied by a second electron, and all of these singly occupied orbitals must contain electrons with the same spin. This means that for a given sublevel with multiple orbitals, such as the p, d, or f sublevels, one electron will go into each orbital until all of them are half-filled before any pairing occurs. To illustrate, if we are populating the p orbitals with electrons, each of the three p orbitals will have one electron with a parallel spin before any second electron is added to one of these orbitals.
Answering the student's question, Hund's rule states that the electrons pair up only after each orbital in a sublevel is occupied by: a) One electron.