The correct answer is A. Periodic variation of load.
Hunting is a self-excited oscillation of the rotor speed of a synchronous motor. It is caused by a mismatch between the motor’s load torque and the torque produced by the motor’s magnetic field. When the load torque is greater than the motor’s torque, the rotor speed decreases. This decrease in speed causes the magnetic field to weaken, which in turn causes the torque to decrease. This cycle repeats, causing the rotor to oscillate in speed.
Periodic variation of load is the most common cause of hunting in synchronous motors. This is because the load torque is not constant, but varies with time. This variation in load torque causes the rotor speed to oscillate.
Over-excitation and over-loading for long periods can also cause hunting, but they are less common causes. Over-excitation occurs when the motor is supplied with more voltage than it is designed for. This causes the magnetic field to become too strong, which can cause the rotor to oscillate. Over-loading for long periods can also cause hunting, because the increased load torque can cause the rotor speed to decrease.
Small and constant load is not a cause of hunting. This is because the load torque is constant, which does not cause the rotor speed to oscillate.