Which type of buoy indicates that it may not be possible to pass on either side?

The indication that a buoy may not be possible to pass on either side is provided by an isolated danger mark. This specific type of buoy is used to signify an underwater hazard that poses a risk to navigation, which could include submerged rocks, wrecks, or other dangerous obstructions. The design of an isolated danger mark typically features distinctive colors—usually black and red—and displays a specific pattern, often with a unique topmark, such as two spherical shapes stacked vertically.

This visual signaling allows mariners to understand that the danger is located close to the buoy, and they must navigate cautiously, treating the area around it with uncertainty. In essence, navigational safety is emphasized by the clear instruction that passing on either side could lead to potential grounding or collision risk.

The other options represent different types of navigational aids. Safe water marks indicate that there are no underwater dangers nearby, preferred channel marks guide vessels through a designated channel, and daymarks of no lateral significance are used for reference but do not indicate hazards. This differentiation highlights why the isolated danger mark is the only one that explicitly conveys the risk associated with proximity to both sides of the buoy.