Final answer:
The question deals with a Keep-Out Mark used in maritime navigation to indicate areas where boats are prohibited; it flashes yellow once every four seconds and is part of the navigational aids that ensure maritime safety.
Step-by-step explanation:
The student's question pertains to navigational aids, specifically a Keep-Out Mark, which signals that boats are prohibited from a certain area. The lights associated with such marks typically follow a flashing sequence, in this case, 'Fl Y (1) 4s,' which means they flash yellow once every four seconds. While the top mark is not applicable or absent in this scenario, identification is often made via letters. Navigational aids like this are essential for maritime safety, guiding vessels and warning of dangers such as shallows, rocks, or protected areas near coastlines or in harbors. Lighthouses also serve a similar purpose, warning ships not to approach too close to hazardous shorelines, with rotating lights that cover all directions, ensuring that the warning is unmistakable.