What should vessels sound to indicate that they are making way in fog?

In conditions of reduced visibility, such as fog, vessels are required to signal their intentions and actions through sound signals. When a vessel is making way, it should sound one prolonged blast at intervals of not more than two minutes. This single prolonged blast serves to communicate to other vessels in the vicinity that the vessel is underway and moving.

The use of a prolonged blast instead of shorter sounds is designed to distinguish the movements of vessels actively navigating through fog from those that are at anchor or drifting, which would use different signaling methods. Thus, sounding one prolonged blast effectively conveys the intended message to other mariners about the vessel's status and helps promote safety in visibility-compromised situations.

The other options do not appropriately convey the message that a vessel is making way. Multiple short blasts or a combination of short and prolonged blasts pertain to different navigational scenarios, such as indicating a vessel's intentions to overtake or signaling its presence without actually moving.