A plaintiff, a citizen of State X, sued a defendant, a citizen of State Y, for negligence in federal district court in State X under diversity jurisdiction, in connection with an automobile accident that occurred in State Y. The defendant has had no contacts with State X. The plaintiff personally served the defendant with a summons and complaint at his home in State Y. The defendant's first response to the complaint was an answer that specifically denied the plaintiff's claims but omitted the defense of lack of personal jurisdiction. Fifteen days after serving the answer on the plaintiff, the defendant amended the answer to include the defense of lack of personal jurisdiction without asking leave of the court.
Which of the following statements is most accurate regarding the defendant's actions?

A: The defense of lack of personal jurisdiction was permanently waived when the defendant's answer failed to include it.
B: The defense of lack of personal jurisdiction has been waived unless the court subsequently grants the defendant leave to amend his answer.
C: The defense of lack of personal jurisdiction was not waived and may be asserted by the defendant.
D: The defense of lack of personal jurisdiction can never be waived and may always be asserted by a defendant.