Final answer:
The standard procedure when noticing a deviation from course while using autopilot is to check for a malfunction to identify any error with the system.
Option C, Check for a malfunction is correct.
Step-by-step explanation:
When steering by autopilot and noticing that the vessel has deviated 15 degrees from the intended course without corrective rudder action, the standard operating procedure to follow first would be to check for a malfunction. This step is crucial to identify whether the autopilot system has encountered an error that results in a failure to apply corrective measures. Once a malfunction is either confirmed or ruled out, subsequent actions can be taken, such as manually adjusting the course or correcting the issue with the autopilot system. Reducing speed or sounding the general alarm may be considered after the initial check, depending on the situation and safety protocols.
Reducing speed (Option a) or sounding the general alarm (Option b) may not be necessary if the deviation is due to a minor glitch or momentary issue with the autopilot. Changing the course manually (Option d) without understanding the cause of the deviation could lead to further complications.
In summary, the initial response when observing a deviation from the course without corrective rudder action while using autopilot is to check for a malfunction (Option c).