The correct answer is: B. its speed is independent of supply frequency.
A synchronous motor is an AC electric motor that runs at a constant speed regardless of the load on the motor. This is because the speed of a synchronous motor is determined by the frequency of the AC power supply and the number of poles on the motor.
A wound-rotor induction motor, on the other hand, has a speed that is slightly less than the synchronous speed. This is because the rotor of an induction motor is not directly connected to the stator, but instead is driven by electromagnetic induction. The speed of an induction motor is therefore determined by the slip, which is the difference between the synchronous speed and the actual speed of the rotor.
The advantage of a synchronous motor over a wound-rotor induction motor is that the speed of a synchronous motor is independent of the load on the motor. This makes synchronous motors ideal for applications where a constant speed is required, such as in conveyor belts and machine tools.
The disadvantages of a synchronous motor are that they are more expensive than induction motors, and they require a DC power supply to excite the rotor.
Option A is incorrect because the power factor of a synchronous motor cannot be varied at will. The power factor of a synchronous motor is determined by the load on the motor.
Option C is incorrect because the speed of a synchronous motor cannot be controlled more easily than the speed of a wound-rotor induction motor. The speed of a synchronous motor is determined by the frequency of the AC power supply and the number of poles on the motor. The speed of a wound-rotor induction motor can be controlled by changing the slip, which is done by changing the resistance of the rotor circuit.