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

Answer:

First option: $\boxed{\uparrow\downarrow}$

Explanation:

Step1: Understand electron - spin representation

Arrows represent electron spins. Up - arrow $\uparrow$ and down - arrow $\downarrow$ indicate opposite spins.

Step2: Analyze each option

In the first option, two electrons in the same box have $\uparrow\downarrow$ which means opposite spins. In the second option, electrons are in separate boxes and not a proper pair in the context of orbital occupancy. In the third option, both electrons have $\uparrow$ (same spin). In the fourth option, the double - up arrows also represent same - spin electrons.