Which of the following actions indicates readiness for the pilot aboard?

The action that indicates readiness for the pilot aboard is raising the square vertical flag. This signal, specifically referred to as the "Pilot Flag," is an internationally recognized symbol indicating that a pilot is on board and ready to take command of the vessel. It communicates to other vessels in the vicinity that the pilot has arrived and is prepared to assist with navigation, particularly in congested or challenging waters.

Other actions, such as furling all sails or dropping anchor, do not specifically convey the presence or readiness of a pilot. Sounding the horn may indicate various situations or requests for attention but is not a standardized signal for pilot readiness. Thus, raising the square vertical flag serves a distinct purpose in maritime communication, ensuring that all vessels are aware of the pilot’s presence and readiness to navigate.