Which vessel light must have the highest visibility?

The masthead light is the correct answer because it is required to be the most visible light on a vessel, particularly during nighttime navigation. According to the COLREGs, the masthead light is positioned at the highest point on the vessel and emits a white light that can be seen from directly ahead and to the sides, typically over a 225-degree arc. This design ensures that vessels can see the masthead light from a considerable distance, which is crucial for avoiding collisions.

The stern light, while also important, has a specific function and is only visible from the rear, thereby having a limited field of visibility compared to the masthead light. All-around lights are visible from a full 360 degrees but are often used on specific types of vessels, such as those at anchor or those that are less than 7 meters in length, which may not provide the same level of visibility on larger vessels as the masthead light. Side lights primarily show the sides of a vessel to oncoming traffic, but they do not have the same range or prominence as the masthead light when it comes to visibility.