Is it A ? Yes ! Two pieces of iron DO always attract each other with equal and opposite gravitational forces, although the forces are so small that we never notice them.
Is it B ? Yes, it could be. If at least one of the pieces of iron is magnetized, then they can attract each other with equal and opposite magnetic forces.
Is it C ? Yes, it could be. But the pieces of iron would have to be joined by a stretched rubber band. Then they would attract each other with equal and opposite elastic forces.
Is it D ? Yes, it could be. If one of the pieces of iron carries a net electric charge, then they attract each other with equal and opposite electrostatic forces.
