If you sight a buoy fitted with a double-sphere topmark at night, it would show what type of light?

A buoy fitted with a double-sphere topmark is designed to indicate a specific type of navigational aid typically used for marking the locations of obstacles or safe passage. The key characteristic of such buoys is their light signal, which serves to alert mariners of their position, especially during low visibility conditions like at night.

The correct option involves a flashing white light that shows a group of two flashes. This pattern is consistent with the signaling characteristics associated with various buoys that have a double-sphere topmark. The purpose of having a specific light pattern is to convey crucial information about the buoy's function and the surrounding navigational challenges.

In this context, the double-sphere topmark is intended to help distinguish the buoy at a distance and assist mariners in identifying critical navigational aids that require attention. Other options describe different light signals not associated with a double-sphere topmark, making them unsuitable for this type of buoy and potentially less effective for nighttime navigation.