One way to understand the role of sleep is to look at ________ happens if we don't get it.

• A. what
• B. that
• C. why
• D. how

Answer: (A)

When you’re saying you want to understand something by examining its consequences, you need a word that names the thing you’re examining. After “look at,” the object should be a clause that describes the events or results. “What happens if we don’t get it” uses what to introduce that clause, meaning “the things that happen.” This makes the sentence natural and clear: you’re looking at the effects of not getting sleep. The other options wouldn’t fit: that isn’t the right way to introduce a clause about the consequences here, while why or how would shift the focus to a reason or manner rather than the events themselves.