In restricted visibility, what must all vessels do according to Rule 19?

Answer: A — Proceed at a safe speed and maintain a proper look-out

In restricted visibility, Rule 19 of the COLREGs requires vessels to take specific actions to ensure safety. The requirement to proceed at a safe speed is crucial because it allows vessels to react in a timely manner to any potential hazards, such as other vessels or obstacles that may not be visible. Maintaining a proper look-out is equally important, as it ensures that the crew is actively monitoring the surroundings for any changes, which is vital when visibility is limited. This approach reflects the need for heightened awareness and caution in unpredictable conditions. By combining safe speed and vigilance through a proper look-out, vessels can reduce the risk of collisions and enhance overall maritime safety during instances of restricted visibility. The other options either do not directly relate to the specific duties outlined in Rule 19 or suggest actions that may not adequately ensure safety in such conditions.