If sleeplessness continues, the part of the brain ________ controls language, memory, planning and our sense of time is affected.

• A. which
• B. that
• C. who
• D. where

Answer: (A)

Choosing the right relative pronoun for a defining clause about a thing. The phrase “the part of the brain” is something (not a person), and the clause “which controls language, memory, planning and our sense of time” adds essential information to identify which part is meant. In this kind of defining clause (no comma before the relative pronoun), using a pronoun for a thing is correct, and “which” is a natural choice in many varieties of English, especially in exam contexts. “Who” would be for people, and “where” for places, so they don’t fit here. (Note that “that” could also be used in many styles, but the sentence as given uses “which.”)