How do isolated danger marks visually differentiate themselves?

Isolated danger marks are specifically designed to indicate hazards that are surrounded by navigable waters. They are visually distinctive to ensure that mariners can easily recognize and safely navigate around these dangers. The primary feature that differentiates isolated danger marks is the use of two black spheres positioned at the top of the buoy. This visual cue is standardized internationally and helps in easily identifying the mark in various weather and lighting conditions.

The presence of the two black spheres at the top signals that there is a hazard below that should be avoided, while the black color itself indicates that it is an isolated danger. This design helps sailors determine the safest routes and avoid collisions or grounding. The other options describe features that do not match the characteristics of isolated danger marks. For instance, red spheres or horizontal stripes are associated with different types of navigational buoys, and flashing lights alone do not provide the clear visual cue needed for these specific hazards.