Answer/Explanation:
18, a,
I believe the n, and s represents North, South, or just the way the arrows are rotating repeatedly.
18, b,
The phenomenon re-occurring shows the alignment of magnetic dipoles with in a magnet, like inside. The arrows show a direction in the line with an external magnetic field in a way that it can produce its own magnetic fields that seem to travel from North to the South.
18, c,
Incorrect. Refrigerator magnets have multiple layers of magnification, they use opposite polarities to stay this way. If he puts the magnet sideways it may stick to the fridge
~ LadyBrain