The correct answer is B. King Duncan's guards.
Macbeth and his wife first killed King Duncan's guards, so that nobody could see what they were about to do. Then they moved on to kill the king himself. After his body was discovered (Macduff was the one to see him first), Macbeth admitted to killing the guards because he thought they were the ones who killed Duncan. He explained he was struck with grief and had to kill them because of their alleged murder.
