Commercial electric motors do not use (c) a permanent magnet to rotate the armature.
Explanation:
Electromagnet usage:
In most commercial electric motors, the rotation is driven by an electromagnet, which is created when current flows through a coil of wire wound around a soft iron core.
Coil design:
The coil has a large number of turns of conducting wire to significantly increase the magnetic field generated when current flows through it.
Soft iron core:
The coil is typically wound around a soft iron core to concentrate the magnetic field and enhance its strength.
Why not permanent magnets:
While some specialized motors might use permanent magnets, commercial motors generally rely on electromagnets for better control and flexibility.
Therefore, option (c) is the correct answer as commercial electric motors typically use electromagnets, not permanent magnets, to rotate the armature.