What is required from vessels navigating in fog according to the Rules?

Answer: B — They must make frequent sound signals

When navigating in fog, vessels are required to make frequent sound signals as a means of signaling their presence to other nearby vessels. Sound signals are crucial in conditions of reduced visibility, as they help alert other mariners to the vessel's location and movements. This practice is outlined in the Rules of the Road to enhance safety and prevent collisions, since visual observations can be significantly impaired in foggy conditions. The other options do not align with the established rules for navigation in fog. Traveling under all sail is not a requirement; devices like engines may also be used. Turning off all lights might further obscure a vessel’s position from others in the dense fog, increasing the risk of accidents. Additionally, increasing speed in such situations is counterproductive, as it can lead to a greater likelihood of accidents in poor visibility. Therefore, making frequent sound signals is the proper action to take when navigating in fog.