Final answer:
Magnetic compasses are highly unreliable in the polar regions where magnetic field lines are perpendicular to the Earth's surface, causing the compass needle to be nearly vertical and not accurately point to magnetic north.
Step-by-step explanation:
Magnetic compasses are highly unreliable in polar regions due to the Earth's magnetic field. The magnetic field lines near the poles are almost perpendicular to the surface, which means that the compass needle can become nearly vertical and fail to point accurately to magnetic north. The correct answer to the student's question is b) Polar Regions.
A compass works by aligning its magnetic needle with the Earth's magnetic field, but at extreme latitudes, especially near the magnetic poles, the needle may not balance well because of the vertical angle (steep inclination) of the magnetic field. As a result, the compass' ability to reliably indicate magnetic north diminishes, which can lead to navigational errors. This phenomenon occurs due to the three-dimensional nature of Earth's magnetic field, which is not uniform over the entire surface of the Earth.