Two red and two overlapping balls in the center are surrounded by a green, fuzzy, circular cloud with a white line running through it. two green balls sit on the white line, and a line leading a bracket around the balls is labeled a. a line leading to a bracket overlapping the white line is labeled b.

a

explanation:

no diagram was provided, but i'm guessing it's the one i've attached to my answer.

here, a mechanical force turns the rod.   this generates an electric current in the coil, which powers the bulb.

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.