Final answer:
A power-driven vessel underway when towing displays specific lights, including a yellow towing light above the sternlight and, for a tow longer than 200 meters, an additional diamond shape and a vertical line of three lights to indicate its status.
Step-by-step explanation:
For a power-driven vessel underway when towing, it's essential to display specific lights or shapes to indicate its status to other vessels, as outlined in the international regulations for preventing collisions at sea (COLREGs). Such a vessel should exhibit a yellow towing light in a vertical line above the sternlight and, if the length of the tow exceeds 200 meters, an additional diamond shape where it can best be seen. When the length of the tow exceeds 200 meters, three lights in a vertical line, the highest and the lowest of which are red and the middle one white, should be shown. These lights help prevent collisions by clearly indicating not only the presence of a tow but also its extensive length if applicable.