You sight a buoy fitted with a double-sphere topmark. If sighted at night, this buoy would show a __________.

Correct Answer: Option B — flashing white light showing a group of two flashes

A double-sphere topmark is the international standard for an isolated danger mark. At night, these marks are required to exhibit a white light with a group-flashing characteristic of two flashes, providing a distinct signal that differentiates the hazard from lateral or safe water marks.