A vessel must proceed at a safe speed __________.

A vessel must always proceed at a safe speed. This principle is foundational in maritime navigation and is emphasized in the International Regulations for Preventing Collisions at Sea (COLREGs). Safe speed is defined as a speed at which the vessel can take proper and effective action to avoid collision and can stop within a distance appropriate to the prevailing circumstances and conditions.

This requirement applies in all situations regardless of visibility, time of day, or congestion. For instance, during periods of reduced visibility, a vessel's capacity to maneuver and respond to the actions of other vessels becomes critical, necessitating a careful control of speed. Similarly, whether navigating at night or in congested waters, maintaining a safe speed ensures that a vessel can react appropriately to other vessels, obstructions, and navigational hazards, thus enhancing safety for all marine traffic. Each scenario mentioned in the options illustrates the importance of safe speed but does not limit its application, reinforcing that safe speed is a constant obligation for all vessels.