My difficulty is in understanding the question stem. The answer seems to fill in a gap, like an assumption family question would require, but the stem is not an assumption family stem. Can anyone help with this?

So I realize that this is an old, old test, but can anyone help me out with the part about the rocks and the magnetic forces in paragraph 2? I'm just not sure what inference should be drawn from it, and this comes into play in Q20. Any help would be really appreciated, thanks!