Visibility is considered "restricted" when it cannot be determined:

Visibility is considered "restricted" when it cannot be determined how far one can see. This definition is essential in navigation and operating a vessel because visibility directly impacts a captain's ability to make safe decisions on the water. Restricted visibility can occur due to fog, rain, snow, or other environmental conditions that limit the distance at which objects, including other vessels, can be seen.

When visibility is limited, it impairs situational awareness, making it difficult for operators to gauge distances and the positions of other boats, buoys, or hazards in the water. Understanding restricted visibility is crucial for implementing proper safety measures, such as reducing speed and sounding appropriate navigational signals, to avoid collisions and ensure the safety of all passengers and crew onboard.

The other options, while related to safe boating practices, do not directly address the concept of restricted visibility. For instance, knowing whether a vessel is moving or if it is properly equipped can be determined without the need for visibility, and the number of vessels present can be ascertained in circumstances where visibility may still be good. Therefore, option B encompasses the critical aspect of what makes visibility restricted.