Final answer:
The mandatory navigation equipment required even under exceptions is a compass, essential for determining direction. While sonar is valuable for locating large underwater objects, it doesn't replace the need for a compass for navigational purposes.
Step-by-step explanation:
The student's question pertains to the requirements for navigation equipment on board vessels, specifically when exceptions to the normal requirements are permitted. The correct answer to the question is a. Compass because it is a fundamental tool for navigation. While other instruments such as GPS and radar enhance navigational capabilities, they are not always mandatory. Sonar is useful for detecting large underwater objects, but it does not provide directional navigation information like a compass does. Sonar technology is valuable for spotting and locating large underwater objects but has limitations in resolving smaller items or detailed shapes. This limitation of sonar makes it unsuitable as a primary navigation tool which is why it is not listed as part of the essential navigation equipment when exceptions are made.
It's important to understand that basic navigation equipment, like a compass, is essential for determining direction when at sea, while other navigational aids such as sonar have more specialized applications such as mapping the seabed or detecting large submerged objects.