Final answer:
The helmsman should reply with "Right 15 degrees" to confirm they are executing the command "Right 15 degrees rudder," as per standard nautical communication procedures.
Step-by-step explanation:
When a helmsman receives the command "Right 15 degrees rudder," the appropriate immediate response is an acknowledgment of the order. Therefore, the helmsman should reply with "Right 15 degrees" to confirm that they have understood and are executing the command. It is a standard nautical procedure for the helmsman to repeat the command they have been given to ensure clear communication and avoid any potential confusion that could arise from misheard or misunderstood orders. This practice helps maintain the safety and efficient navigation of the vessel.