What lights must a 50-meter power-driven vessel have when pushing a 210-meter group of barges?

A power-driven vessel that is pushing a group of barges, especially one as lengthy as 210 meters, must exhibit specific light configurations to ensure safe navigation and visibility. According to the COLREGs (International Regulations for Preventing Collisions at Sea), when a vessel is engaged in towing or pushing, it must display additional masthead lights to signal its length and ability.

The correct answer indicates that the vessel must display two masthead lights, with one additional masthead light located on the other mast. This setup is necessary for vessels pushing or towing multiple barges, as it extends the visual perception of the vessel's length, allowing other mariners to better gauge its size and navigation path. Additionally, sidelights and a sternlight are required to indicate the vessel's side and rear, which is important in preventing collisions.

To summarize, the two masthead lights plus the additional one from the second mast cater to the overall length of the vessel when engaged in pushing a large configuration of barges, fulfilling the COLREGs requirements for such navigational scenarios.