What must every vessel do in conditions of restricted visibility according to Rule 19c?

In conditions of restricted visibility, Rule 19 of the COLREGs emphasizes that every vessel must have due regard to the prevailing circumstances and conditions of visibility. This means that vessels are required to take into account the surrounding environment, which includes factors such as fog, rain, or any situation that may impair visibility. The regulation mandates that vessels ensure they navigate safely, considering potential hazards and the presence of other vessels.

Maintaining due regard indicates that vessels should adjust their speed, course, and maneuvers based on the limitations imposed by restricted visibility to avoid collisions. This is crucial because restricted visibility increases the risks of accidents, and vessels need to be more vigilant and responsive to the situation around them.

In contrast, the other choices fail to reflect the importance of situational awareness and adjustments needed in such conditions. Ignoring conditions while using radar does not account for the potential limitations of radar systems, which may not provide complete situational awareness. Limiting action only to when other vessels are present neglects the possibility of unseen obstacles. Lastly, maintaining a constant speed regardless of conditions could lead to dangerous scenarios, as it disregards the necessity to slow down or alter course in response to visibility challenges.