Under the U.S. Aids to Navigation System, a lighted buoy with a spherical topmark marks ________.

A lighted buoy with a spherical topmark signifies safe water within the U.S. Aids to Navigation System. This type of buoy is intended to indicate that vessels can navigate in the waters around it without encountering dangerous underwater obstacles or hazards. The spherical topmark is a specific identifier that conveys this meaning, distinguishing it from other buoy types that indicate different navigational information.

The context of a safe water buoy is essential for maritime navigation, as it provides clear guidance to mariners about areas that are free of significant hazards. This allows them to safely navigate through waters, particularly in places where there may be shallow areas or other potential dangers that are not marked by different buoy types.

Other options, such as marking a fish trap, a hazard to navigation, or a bifurcation in the channel, correspond to different types of buoys or navigational aids that do not convey the same meaning as safe water. Each of those options serves a specific function in aiding navigation, but the spherical topmark distinctly indicates a safe route for vessels, making it a critical element for ensuring the safety of maritime operations.