The correct answer is: A. the force required to accelerate the rotor to the synchronous speed in an instant is absent.
A synchronous motor is an AC motor that runs at a constant speed that is synchronized with the frequency of the AC power supply. The rotor of a synchronous motor is made of a permanent magnet or a wound rotor with a DC current applied to it. The stator of a synchronous motor has a three-phase winding that is energized with AC power. The rotating magnetic field produced by the stator winding induces a current in the rotor winding. This current creates a magnetic field that interacts with the stator field to produce a torque that drives the rotor.
The rotor of a synchronous motor cannot start by itself because the torque required to accelerate the rotor to synchronous speed is very large. This torque is proportional to the square of the slip, which is the difference between the synchronous speed and the rotor speed. The slip is zero when the rotor is at synchronous speed, so the torque required to accelerate the rotor to synchronous speed is infinite.
To start a synchronous motor, a starting device is used to accelerate the rotor to near synchronous speed. Once the rotor is near synchronous speed, the stator field induces a current in the rotor winding that creates a magnetic field that interacts with the stator field to produce a torque that drives the rotor to synchronous speed.
Options B, C, and D are incorrect because they do not explain why a synchronous motor is not inherently self-starting.