The correct answer is: A. When supply voltage fluctuates.
Synchronous motors are designed to operate at a constant speed, which is determined by the frequency of the power supply and the number of poles on the stator. If the supply voltage fluctuates, the speed of the motor will also fluctuate. This can cause the motor to run at a speed that is not synchronized with the power grid, which can lead to problems such as overheating and damage to the motor.
Option B is incorrect because load variation does not cause hunting in a synchronous motor. In fact, load variation can actually help to stabilize the speed of the motor. This is because the load on the motor creates a torque that opposes the change in speed. This torque helps to keep the motor running at a constant speed, even when the load is changing.
Option C is incorrect because power factor is not a factor that affects hunting in a synchronous motor. The power factor is a measure of how efficiently the motor is converting electrical energy into mechanical energy. However, it does not affect the speed of the motor or the likelihood of hunting.
Option D is incorrect because a motor that is under loaded is less likely to hunt. This is because the load on the motor creates a torque that opposes the change in speed. When the load is low, the torque is also low, which makes it more likely that the motor will hunt.
In conclusion, the correct answer is A. When supply voltage fluctuates.