The correct answer is: A. remains constant at all loads.
The field flux in a 3-phase synchronous motor is produced by the field windings, which are located on the stator. The field windings are energized by a DC voltage, which creates a magnetic field that rotates at a constant speed. The rotor of the motor is made of a ferromagnetic material, and it is attracted to the rotating magnetic field. As the rotor rotates, it aligns itself with the magnetic field, and this causes the motor to turn.
The magnitude of the field flux is determined by the current in the field windings. The current in the field windings is constant, so the magnitude of the field flux is also constant. This means that the speed of the motor is also constant, regardless of the load.
Option B is incorrect because the field flux does not vary with speed. The speed of the motor is determined by the frequency of the AC power supply and the number of poles on the stator. The field flux does not affect the speed of the motor.
Option C is incorrect because the field flux does not vary with load. The load on the motor affects the torque that the motor produces, but it does not affect the speed of the motor or the magnitude of the field flux.
Option D is incorrect because the field flux does not vary with power factor. The power factor is a measure of how efficiently the motor converts electrical energy into mechanical energy. The power factor does not affect the speed of the motor or the magnitude of the field flux.