What indicates the absence of the civil or military official whose flag is flying on the ship?

The correct answer reflects the standardized protocol for signaling the absence of a civil or military official aboard a vessel, particularly when their flag is displayed. The "Fourth substitute" is specifically designated for this purpose. When a ship flies the flag of a particular official and subsequently signals their absence, the Fourth substitute (which is typically a flag or pennant representing that absence) is displayed.

This practice is essential in naval and maritime operations as it communicates critical information to other vessels and personnel about the status of individuals in command or holding significant positions. By using this signaling system, ships maintain effective communication and protocol adherence while at sea, ensuring all crew members and outside parties are informed about the command structure onboard.