A system initially consists of a ball of mass mat rest at height h above the Earth and the Earth, of mass Mę. When the ball has fallen a distance h/2, the total momentum of the system is:

the answer is the first electromagnet (a). this is because it has more coils around the ferromagnetic material. increasing the number of coils increases the surface area in which the metal interacts with the current in the coil. this enables more atoms in the metal to become aligned hence becoming more magnetized.