A buoy marked with a composite group flashing light indicates what?

A buoy that displays a composite group flashing light indicates a preferred channel for vessels to navigate through. This type of light pattern is designed to mark an area where it is safest for vessels to proceed, providing a clear indication of the best route to take for safe navigation. The visual cue helps mariners quickly identify the preferred path, ensuring they stay on course while avoiding potential hazards or obstacles.

By recognizing that the composite flashing light signifies a preferred channel, mariners can make informed decisions to ensure safer passage within waterway systems. Other options do not convey this specific navigational aid function, as they represent different meanings related to safety and navigation warnings or restrictions.