Which situation provides the best evidence that a chemical reaction is taking place? A).A chemical completely dissolves in water and the temperature of the solution remains constant
B).A metal strip bends when heat
C).Two solutions mixed produce a solid
D).A solution freezes in the freezer

see earlier

an electric circuit

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.