The report was written by a scientist ______ studies climate change.

• A. That
• B. Which
• C. Who
• D. Whom

Answer: (C)

This tests using a relative pronoun for a person who is the subject of the clause that follows. The blank must introduce a phrase describing the scientist, and that phrase contains its own verb: studies climate change. The pronoun here functions as the subject of that verb, so the natural choice is the person-relative form used as the subject: who. “Which” is for things, and “whom” is the object form (it would only be correct if the scientist were the object of another verb in the relative clause). While “that” can sometimes replace “who” in informal writing, the standard, most natural choice in this context is who. So the sentence reads: The report was written by a scientist who studies climate change.