Which vessel is not required to show a light in the absence of an engine?

Answer: B — vessel aground

A vessel aground is not required to show a light in the absence of an engine because the regulations state that when a vessel is aground, it does not have to display lights, except when it is in motion. The primary purpose of displaying lights is to ensure that vessels can see each other and avoid collisions. When a vessel is aground, it is stationary and not engaged in activity that would pose a risk of collision with other navigational traffic. In contrast, a sailing vessel under power, a vessel at anchor, and a fishing vessel all have specific lighting requirements to enhance visibility and ensure safe navigation. For example, a sailing vessel under power must display lights indicating its status, and an anchored vessel has to show an anchor light. This is to inform other vessels of their presence and status on the water to avoid any potential collisions.