Which of the following vessels is required to display specific lights according to Rule 21?

The requirement for vessels to display specific lights according to Rule 21 pertains to certain types of vessels, particularly those that are engaged in fishing. This rule outlines the definitions and classifications of different types of vessels and sets specific lighting requirements based on their activities.

Fishing vessels engaged in fishing are required to display distinct lights to ensure they are easily identifiable by other vessels. This is crucial for safety and helps prevent collisions, especially in areas where fishing activities are prevalent. By displaying these specific lights, fishing vessels signal their activity and status to others on the water, which is essential for maintaining navigational safety and awareness.

The other options imply a broader or different classification that does not align with Rule 21's specific requirements. For example, not all vessels are subject to the same lighting regulations, as different types of vessels, like cargo and passenger ships, follow other specific rules regarding display lights depending on their operation and size. Therefore, the focus on fishing vessels in this context illustrates the importance of adhering to tailored regulations designed for particular activities at sea.