After one night without sleep, concentration becomes difficult. If sleeplessness continues, the part of the brain ________ controls language, memory, planning and our sense of time is affected.

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

Answer: (D)

This item tests choosing the right relative pronoun to introduce a defining clause about a thing. The noun being described is non-human and singular: “the part of the brain.” For a noun like this, use which or that to introduce essential information about it. The other options—who, where, when—show you’re talking about a person, a place, or a time, not a body part, so they don’t fit the meaning here. Because the clause “which/that controls language, memory, planning and our sense of time” is essential to identify which part is being talked about, it’s a restrictive clause and no comma is used. So the best fit is which/that.