Final answer:
Boats over 9 meters must be equipped with distress signals, navigation lights, and sound signaling devices to ensure safe navigation and effective communication, especially in emergencies or poor visibility. Navigation lights indicate a vessel's presence and orientation, while distress signals and sound signaling devices like sonar help with safety and navigation.
Step-by-step explanation:
The correct answer to the question regarding what visual signals boats over 9 meters must have is d. Distress signals, navigation lights, and sound signaling devices. It is mandatory for such vessels to be equipped with these in order to ensure they can communicate effectively in times of distress and navigate safely, especially at night or in poor visibility conditions. Navigation lights help other vessels to determine the size, type, and direction of the boat, which is essential to prevent collisions. Moreover, distress signals serve to signal others in the event of an emergency, and can be visual like flares, or auditory like sound signals or mayday calls. Lastly, sound signaling devices, which include horns, bells, and whistles, are used for communication, especially in instances where visibility is restricted, such as in fog or heavy rain.
Moreover, the use of sonar (Sound Navigation And Ranging) is an important navigational tool used by ships to determine the depth of the ocean by sending out a sound wave that bounces off the seabed. The time it takes for the sound to travel to the seabed and back to the ship is measured, and because the speed of sound in water is known, this time can be used to calculate the depth. This method exemplifies how sound signaling devices not only are important for communication but also for navigation and safety at sea.