Question

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

Explanation:

Electrons in the same orbital with opposite spins are represented by one upward - pointing arrow ($\uparrow$) and one downward - pointing arrow ($\downarrow$) in the same box (orbital).

Answer:

The first option (with $\uparrow\downarrow$ in the same box) shows a pair of electrons that have opposite spins.