Which vessel must exhibit forward and after masthead lights when underway?

The requirement for a vessel to exhibit forward and after masthead lights when underway is primarily specified in the International Regulations for Preventing Collisions at Sea (COLREGs). According to these regulations, power-driven vessels of 12 meters or more in length must show a masthead light at the front and another masthead light positioned at the stern when they are in motion.

In this context, a 50-meter power-driven vessel is significantly larger than the threshold of 12 meters, thereby mandating the display of both forward and after masthead lights. This configuration helps other vessels to identify the type of vessel and its direction of travel during nighttime or low-visibility conditions.

When assessing the other options, a sailing vessel under 20 meters is not required to display both masthead lights, as lighter obligations may apply. Similarly, a 30-meter sailing vessel and a 15-meter fishing vessel, being vessels under certain lengths, are also not required to exhibit these specific lights while underway. Power-driven vessels over a specific size, particularly 50 meters as in the correct answer, follow distinct light display requirements to ensure safe navigation and visibility on the water.