The correct answer is D. Synchronous motor.
A synchronous motor is an AC motor that runs at a constant speed, synchronized with the frequency of the AC power supply. It has a stator with windings that are energized by an AC power source, and a rotor that has permanent magnets or a wound rotor with DC current applied. The stator field creates a rotating magnetic field that induces a current in the rotor. The rotor current interacts with the stator field to produce a torque that turns the rotor at the same speed as the stator field.
A permanent magnet motor is a type of DC motor that uses permanent magnets to create a magnetic field. The stator of a permanent magnet motor has windings that are energized by a DC power source. The current in the stator windings creates a magnetic field that interacts with the magnetic field of the permanent magnets to produce a torque that turns the rotor.
A series motor is a type of DC motor that has the field windings and the armature windings connected in series. This means that the same current flows through both windings. The field windings create a magnetic field that interacts with the magnetic field of the armature to produce a torque that turns the rotor.
A squirrel cage motor is a type of AC induction motor that has a squirrel cage rotor. The rotor consists of a series of bars that are connected by short-circuiting rings. The stator of a squirrel cage motor has windings that are energized by an AC power source. The current in the stator windings creates a rotating magnetic field that induces a current in the rotor bars. The current in the rotor bars interacts with the stator field to produce a torque that turns the rotor.
Of the four motors listed, only the synchronous motor is not suitable for use as a DC machine. This is because a synchronous motor requires an AC power source to operate.