A power-driven vessel towing astern shall show __________.

A power-driven vessel that is towing astern is required to show a towing light in a vertical line above the sternlight. This specific arrangement helps identify the type of vessel and its activity to other mariners, which is critical for safe navigation and collision avoidance. The towing light is typically yellow and visually distinguishes the towing operation from other types of vessels.

Displaying the lights as specified in the regulations enhances the visibility and understanding of the vessel's intentions and maneuvers, ensuring that other vessels can react appropriately to the presence of the towing vessel. This is crucial for operational safety, especially in busy or constrained waterways where multiple vessels may be navigating in close proximity.

Showing two masthead lights, presenting no lights, or exhibiting fishing lights would not provide the necessary information about the vessel's operation, thereby compromising safety on the water and contradicting the established convention for navigation lights. The correct configuration ensures clarity and compliance with maritime navigation rules.