The correct answer is: B. flux density.
The back emf in the stator of a synchronous motor is the induced emf that opposes the applied voltage. It is proportional to the flux density, which is the magnetic field strength in the stator. The flux density is determined by the number of stator poles, the rotor speed, and the rotor excitation.
The number of stator poles determines the number of magnetic poles in the stator. The more poles, the stronger the magnetic field.
The rotor speed determines the frequency of the magnetic field in the stator. The faster the rotor speed, the higher the frequency of the magnetic field.
The rotor excitation determines the strength of the magnetic field in the rotor. The stronger the magnetic field, the stronger the back emf.
The back emf is important because it determines the current that flows in the stator. The current that flows in the stator determines the torque that is produced by the motor.