What is the light phase characteristic of a lighted isolated-danger mark?

The light phase characteristic of a lighted isolated-danger mark is indeed Group flashing. This type of light is specifically designed to indicate isolated dangers to navigation, such as rock formations or underwater wrecks. The Group flashing characteristic consists of a series of flashes in a group, typically showing a specific number of flashes followed by a period of darkness. This allows mariners to visually recognize that they are navigating near a potential hazard and helps them to maintain a safe distance.

The Group flashing light is distinct and easily recognizable among other navigation lights, enhancing safety by providing a clear signal about the nature of the danger. In contrast, characteristics like interrupted quick flashing, very quick flashing, or long flashing do not convey the specific meaning of an isolated-danger mark and are instead associated with other types of navigational aids. It's essential for mariners to understand these characteristics to interpret the signals displayed by lighted marks accurately.