Which light is specifically defined as showing an unbroken light over an arc of the horizon of 225 degrees?

Answer: C — Masthead light

The masthead light is defined as projecting an unbroken light over an arc of the horizon of 225 degrees. This specific characteristic is crucial for nighttime navigation and safety, as it allows vessels to be seen from both the front and side, enhancing visibility to other vessels and indicating the boat's orientation and direction of travel. The masthead light is typically mounted at the fore part of a vessel and is required by the COLREGs for vessels under way, ensuring that the combined lights from the front and sides create a clear visual representation of the vessel’s heading. This light is also positioned higher than the sidelights, allowing it to be visible even from considerable distances, which is essential for avoiding collisions in congested waters.