What is the requirement for vessels when navigating in dense fog according to the Navigation Rules?

When navigating in dense fog, the requirement is for vessels to use sound signals and take precautionary measures to ensure safety. This is crucial because visibility is severely reduced in foggy conditions. Using sound signals, such as horn blasts, alerts other vessels to the presence of your own, helping to prevent collisions. Additionally, vessels are encouraged to slow down, be vigilant, and take extra care while navigating, allowing for better reaction times and increased awareness of their surroundings.

The other choices do not align with the safety protocol required in such conditions. For instance, speeding up could lead to dangerous situations, as it reduces reaction time and control. Maintaining a constant course and speed may not be safe if a vessel is approaching another craft or obstacle. Turning off navigation lights would impair visibility for other vessels, which is counterproductive to the goal of avoiding collisions. Thus, utilizing sound signals and taking appropriate safety precautions are essential actions when navigating through dense fog.