Answer:
The reference point in an electrical circuit to measure voltages is called Earth or Ground; that point is a direct hookup to earth that helps stabilizing electrical currents to prevent high voltage surges.
What makes a substance easily grounded is to be a good energy conductor; metallic objects are generally good energy conductors aka GEM "Ground Enhancement Materials" and they are mostly produced from rocks and minerals. That's why the answer is (B.) Iron Nail.
Explanation:
