The correct answer is: C. an alternator.
A synchronous motor is an AC electric motor that rotates at a speed that is synchronized with the frequency of the AC power supply. The construction of a synchronous motor resembles that of an alternator, with the addition of a rotor winding that is connected to an external DC power supply. The rotor winding creates a magnetic field that interacts with the stator winding to produce torque.
A series motor is an AC electric motor that has the stator and rotor windings connected in series. The current through the stator and rotor windings is the same, which results in a strong magnetic field that produces a lot of torque. However, series motors are inefficient and have a poor power factor.
An induction motor is an AC electric motor that has the stator windings connected to an AC power supply and the rotor windings left unpowered. The stator windings create a rotating magnetic field that induces a current in the rotor windings. The current in the rotor windings creates a magnetic field that interacts with the stator field to produce torque. Induction motors are the most common type of AC motor.
A rotary converter is a device that converts AC power to DC power or vice versa. Rotary converters are not commonly used today, as they have been replaced by more efficient solid-state devices.