Answer: C. Antagonist.
Step-by-step explanation: An antagonist is a person (in this case a character) who actively opposes or is hostile to someone or something, we can also say that it is an adversary. In the given excerpt we can see that Steven represents the antagonist, because when he arrives, he damages the friendship between Mick and Alec, by spreading lies about Mick and convincing Alec that Mick wasn't a good friend.