The correct answer is C) "Right 15 degrees rudder". When a helmsman receives a command to change the rudder angle, the proper response is to repeat the order verbatim. This verbal acknowledgment confirms that the helmsman has heard and understood the command. By repeating the order, the helmsman demonstrates that they are ready to execute the maneuver as directed. The other options are incorrect because: A) No reply is necessary - this is incorrect, as a verbal acknowledgment is required. B) "Rudder is right 15 degrees" - this does not exactly match the order given. D) "Aye Aye Sir" - this is a more general acknowledgment, rather than a specific repeat of the order.