A lantern combining a vessel's navigation lights may be shown on a __________.

A lantern that combines a vessel's navigation lights can indeed be displayed on a vessel less than 12 meters in length, as stated in the COLREGS (International Regulations for Preventing Collisions at Sea). The 15-meter sailing vessel is an appropriate choice because vessels of this length can use a combined lantern to show their navigation lights, streamlining the light display for efficiency and clarity.

Vessels typically over 12 meters are required to display separate navigation lights to ensure proper identification of their status and type. This is crucial for the safety of navigation and helps in reducing the risk of collisions. Larger vessels, such as the 20-meter fishing vessel and the 25-meter power-driven vessel, follow this requirement and would not use a combined lantern.

The specific regulations for a pilot vessel also necessitate distinct navigation light displays, aligning with the broader goal of ensuring safe navigation by making each type of vessel easily identifiable to other mariners. Thus, the selection of a combined lantern is limited to smaller vessels, reinforcing why a 15-meter sailing vessel is the correct answer for this scenario.