The correct answer is D. all of the above.
The back emf of a synchronous motor is the induced emf in the rotor that opposes the applied voltage. It is proportional to the speed of the rotor and the load angle. The load angle is the angle between the rotor current and the stator field. The back emf is important because it determines the torque of the motor.
The back emf is proportional to the speed of the rotor because the faster the rotor turns, the more lines of flux it cuts per unit time. This induces a larger emf in the rotor.
The back emf is also proportional to the load angle because the larger the load angle, the more current flows in the rotor. This current creates a magnetic field that opposes the stator field. This opposition reduces the back emf and the torque of the motor.
Therefore, the back emf of a synchronous motor depends on speed, load, and load angle.