If a rowboat underway does NOT show the lights specified for a sailing vessel underway, it shall show which of the following?

The correct answer emphasizes the requirement for a rowboat that is underway to exhibit a white light to enhance visibility and avoid collisions. According to the International Regulations for Preventing Collisions at Sea (COLREGs), vessels that don’t fit into the defined categories must still ensure they are visible to others, particularly in low-light conditions.

A white light helps in signaling the rowboat's presence to other vessels, allowing them ample time to avoid potential collisions. This provision is particularly important because rowboats typically do not meet the standards for displaying sidelights as required for sailing vessels.

The other options present scenarios that do not align with the regulations governing visibility and safety for vessels on the water. Displaying only a stern light or sidelights would not adequately inform other vessels of the rowboat's position and movement, especially during times of decreased visibility. Not showing any lights at all would significantly increase the risk of collision, thus the regulations necessitate at least the presence of a white light to maintain safety on the water.