Final answer:
Vessel operators should reduce speed when approaching a narrow channel, a crossing situation, and a vessel not under command to ensure safety and avoid collisions or accidents. Thus, option d) all of the above is the correct answer.
Step-by-step explanation:
Vessel operators should reduce speed when approaching d) all of the above, which includes: a) a narrow channel, b) a crossing situation, and c) a vessel not under command.
Reducing speed in these situations is important to ensure safety and avoid potential collisions or accidents. In a narrow channel, reducing speed allows for better maneuverability and gives time to react to any obstructions or other vessels.
In a crossing situation, reducing speed allows for better visibility and ensures that the vessel can safely navigate the crossing. When approaching a vessel not under command, reducing speed gives the operator more time to assess the situation and avoid any potential hazards.