B

The magnetic compass works by aligning its needle with the Earth's magnetic field.
In the regions near the magnetic poles, the magnetic field lines are almost vertical.
This means that instead of pointing horizontally as they do near the equator, the compass needle will tend to dip down towards the Earth's surface.
Consequently, the compass becomes difficult to read, as it does not point clearly in a horizontal direction that is useful for navigation.
Therefore, the correct answer is option B.