In Rule 2, which precaution is mentioned as necessary by 'ordinary practice of seamen'?

The correct choice highlights the importance of maintaining safe navigation practices and respecting the hierarchy of vessel movements as outlined in the COLREGs. When the rule emphasizes that a vessel underway should keep out of the way of a vessel at anchor, it underlines the principle that vessels that are actively moving should take extra caution to avoid interfering with the operations and safety of stationary vessels.

This precaution is necessary because a vessel at anchor is typically not in a position to maneuver or respond to the presence of other vessels approaching it. By having vessels underway prioritize the safety of those at anchor, it promotes an organized and safe navigation environment, reducing the risk of collisions and ensuring that both types of vessels can operate effectively within their proper contexts.

The other options reflect situations that do not align with established navigation conventions or may introduce unnecessary risks in maritime safety practices. Thus, the significance of the chosen precaution stems from the need for conscientious adherence to established maritime rules to ensure the utmost safety on the waterways.