What action should an airplane take if the assistance of the vessel is no longer required?

The appropriate action for an airplane when it no longer requires the assistance of a vessel is to cross the vessel while close astern at a low altitude while rocking the wings. This maneuver serves multiple purposes: it allows the airplane to signal to the vessel that it is safe to depart and that assistance is no longer needed, while also ensuring that the airplane maintains a safe distance from the vessel. Rocking the wings is an internationally recognized gesture of thanks and communication, indicating to the vessel's crew that the airplane is acknowledging their assistance.

This action reinforces safety by keeping the airplane visible and clear of the vessel while showing gratitude. It is essential in maintaining good communication practices in aviation and maritime operations since misunderstandings can lead to dangerous situations. The low altitude helps ensure that the vessel crew can clearly observe the airplane's actions.

The other options do not facilitate appropriate visual communication or safe separation from the vessel, which is critical when transitioning from a situation requiring assistance to one where the airplane can continue its flight independently.