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

In conditions of restricted visibility, Rule 19c emphasizes the importance of vessels having due regard to the prevailing circumstances and conditions. This means that vessels must remain vigilant and take into account the specific environmental factors at play, such as reduced visibility due to fog, rain, or other obstructions.

This rule is crucial for ensuring safe navigation, as it requires mariners to adjust their actions based on visibility and potential risks, rather than following a rigid set of actions. For example, a vessel may need to reduce speed, alter course, or take additional precautions when encountering fog, rather than proceeding as if visibility was normal. The intention behind this rule is to prioritize safety at sea, making clear that vessels must remain adaptable to changing conditions.

The other choices do not align with the requirements set by Rule 19c. For instance, ignoring conditions simply because radar is in use disregards the potential for radar limitations in poor visibility. Similarly, only following rules when other vessels are present undermines the need for proactive safety measures in all situations. Maintaining a constant speed regardless of conditions could lead to dangerous situations, as it does not account for the need to adapt to specific circumstances that may require a change in navigation strategy.