During poor visibility, what must vessels do according to the Rules?

Answer: C — Display navigation lights and proceed at a safe speed

In conditions of poor visibility, the Rules of the Road require vessels to display their navigation lights and proceed at a safe speed. This is crucial because navigation lights help other vessels to identify your presence and position, allowing for safer navigation among potentially unseen vessels. Proceeding at a safe speed is essential as it enables a vessel to take proper action, including stopping, if necessary, to avoid collisions. Safe speed is defined as a speed that allows a vessel to maneuver safely in the prevailing conditions, which is particularly important when visibility is compromised. The other options do not align with the requirements set forth by the Rules. Stopping navigation completely may not be practical or safe, as vessels still need to move to avoid hazards. Traveling at full speed while sounding signals does not enhance safety, as it increases the risk of collision. Lastly, using visual signals instead of lights contradicts the specific requirements for navigation lights in poor visibility, as lights are essential for safety in such conditions.