A vessel is towing a partly submerged object with a length greater than 100m and a breadth greater than 25m. How should it be lit?

The appropriate lighting for a vessel that is towing a partly submerged object longer than 100 meters and broader than 25 meters is to display two towing lights and a stern light.

According to the International Regulations for Preventing Collisions at Sea (COLREGs), vessels engaged in towing operations must show specific lights to provide visual indicators of their actions and ensure safety. The two towing lights are positioned vertically and indicate the direction of the towing operation, while the stern light provides visibility from the rear of the vessel. This arrangement effectively communicates the size and nature of the vessel's operation to other vessels, which is essential for safe navigation.

The other options do not meet the specific requirements set forth in the COLREGs for vessels towing objects of that size. The single towing light or all-round white light does not convey enough information about the towing arrangement, while three masthead lights in a vertical line are not used for this situation and do not serve the purpose of indicating a towing vessel properly. Hence, the combination of two towing lights and a stern light is the correct and safe method to ensure that the towing vessel is rendered conspicuously visible to other maritime traffic.