Final answer:
One benefit of having a magnetic compass on board vessels is its ability to provide instant heading information. Therefore correct answer is option b.
Step-by-step explanation:
An advantage of the magnetic compass aboard vessels is that it provides instantaneous heading information. When a compass is placed on a vessel, the needle aligns with the Earth's magnetic field direction and points north-south. This allows sailors and navigators to determine their current heading and make informed decisions about their course.
While the compass is a valuable tool for navigation, it is important to note that it is not exact as it shows magnetized north and not true north. Gyroscopic errors and the presence of magnetic fields can also affect compass readings, so sailors must take these factors into account when using a compass aboard vessels.
Overall, the magnetic compass is a reliable and essential instrument for navigation, especially when other modern technologies may not be available or fail.