“Who” and “whom” are often confused, and the ACT knows this, so it will test it. “Who” is a subject pronoun for something doing the action. “Whom” is the object pronoun for something receiving the action. (To help you remember, remember that the object pronoun is the one with the “m”, just like the object pronouns "him" and "them").
Examples: My favorite science professor is the teacher who is going to perform in the talent show. Who is going to watch? I don’t know whom the students voted as the winner. Whom did you vote for?