The correct answer is B: The murder of Polonius by Hamlet
This plot event accounts for the change in Laertes's character. At the beginning of the play, Laertes is a calm, gentle and docile person. However, after his father, Polonius, is murdered by Hamlet he returns to Denmark in order to avenge his father's murder. So, we can see that from Act I to Act IV Laertes's character changes dramatically as he becomes vengeful and he isn't a law-abiding citizen any more.