Which light indicates the back of the vessel?

The correct choice identifies the stern light as the light that indicates the back of the vessel. The stern light is specifically designed to be located at the rear of a boat and is typically white. Its primary function is to make the vessel visible to others from behind, helping to prevent collisions during nighttime or low-visibility conditions. This light plays a crucial role in navigational safety, as it allows other vessels to determine your vessel's orientation and direction of travel.

The masthead light is located at the front of the vessel and helps indicate its presence and heading from the front, while side lights are positioned on each side of the vessel to show a colored light when viewed from different angles. The anchor light is used to indicate that a vessel is anchored and is typically positioned higher than the other lights. Each of these lights has its own specific purpose, but the stern light is the only one that specifically indicates the rear of the vessel.