What shape does an "isolated danger mark" take?

An isolated danger mark is a specific type of marker used in the IALA Maritime Buoyage System to indicate a hazard that may be completely surrounded by navigable waters. These marks can take two forms: cylindrical and conical. The purpose of the shape is to provide clear visual identification to mariners, allowing them to recognize the danger in various environmental conditions and from different distances.

The cylindrical shape typically features a flat top, while the conical shape generally has a pointed top, both of which are designed to be easily distinguishable from other types of navigation aids. The ability for these markers to utilize either shape serves to create flexibility in buoy design, accommodating local navigational challenges and assisting sailors in identifying hazards correctly.

The other choices focus on a singular shape or an unusual design that does not align with the established characteristics of an isolated danger mark, which are specifically recognized for their cylindrical and conical forms in the IALA system. This ensures that mariners can rely on a consistent signaling system in waterway navigation.