Three positive charges a, b and c, and a negative charge d are placed in a lines as shown in the diagram. all four charges are of equal magnitude. the distances between a and b , b and c, and c and d are equal.

the part of the wave he is travelling on is the crest

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.