According to Rule 19d, what should a vessel avoid if altering course?

The correct choice emphasizes the importance of maintaining safe navigation and preventing collisions in accordance with Rule 19d, which pertains to conduct in restricted visibility. When a vessel alters its course, it should be cautious about making a turn to port if another vessel is ahead of its beam. This is primarily to avoid creating a situation that could result in a collision, as turning to port may bring the two vessels into closer proximity, especially when visibility is limited.

In marine navigation, it is crucial to consider the positions and movements of other vessels in relation to your own. By avoiding a port turn when another vessel is directly ahead or nearly so, the risk of an encounter is minimized. This principle encourages vigilance and responsible decision-making, ensuring that vessel operators are aware of potential risks and are acting in a manner that promotes safety.

The other options involve situations that may not adhere to the principles outlined in the rules regarding safe navigation or do not directly address the risk of collision related to the action of altering the vessel's course.