Question

which diagram shows a pair of electrons that have opposite spins?

Explanation:

In quantum - mechanics, electrons in an orbital can have two opposite spins, which are represented by up - arrow ($\uparrow$) and down - arrow ($\downarrow$). A pair of electrons with opposite spins is shown when one electron is represented by an up - arrow and the other by a down - arrow in the same orbital (represented by a box).

Answer:

The first option with two arrows (one up and one down) in a single box shows a pair of electrons with opposite spins.