Richard Linklater, _____ directs films about complicated personal relationships, faced a unique challenge in casting the actors and actresses for his film Boyhood.

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

Answer: (A)

When describing a person with extra information, use a nonrestrictive relative clause and a subject pronoun. Here, the phrase added after the name explains more about who he's like as a director: “who directs films about complicated personal relationships.” The pronoun needs to be the subject of that clause, which is why the correct choice is who. It refers to Richard Linklater and is the one doing the directing in the relative clause: he directs films. Using whom would make the pronoun the object, which doesn’t fit the structure of the clause. Which would imply a thing, not a person, and that doesn’t describe Richard Linklater. That is typically reserved for restrictive statements, and with the extra information set off by commas, that usage would sound off.