A vessel under 50 meters in length must display how many mast lights?

A vessel under 50 meters in length is required to display one mast light at the highest point of the vessel or near its position on the mast if one is installed. This single mast light serves as an important navigational aid, indicating the presence of the vessel to other ships during nighttime or low visibility conditions. The requirement helps ensure safety at sea by making vessels more visible to each other, allowing for safer navigation and collision avoidance.

The other choices do not reflect the regulations set out in the International Regulations for Preventing Collisions at Sea (COLREGs) or the associated guidelines for vessels of this size. For longer vessels, additional mast lights may be needed to denote their length and capability, but that does not apply within this specific length category.