When a pupil plays her flute in the classroom the window vibrates. Why?

he sound waves travel to the window in the form of vibrations of air molecules. The air molecules near to the window receive the vibration and hit the window thereby passing on on the vibration.

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.