The correct answer is: C. the rotating stator flux and rotor poles.
The torque angle is the angle between the rotating stator flux and the rotor poles. When the rotor poles are aligned with the rotating stator flux, the motor is in synchronism and produces maximum torque. As the rotor poles lag behind the rotating stator flux, the torque decreases. When the rotor poles are 90 degrees behind the rotating stator flux, the motor is at standstill and produces no torque.
The supply voltage and the back emf are always in phase with each other in a synchronous motor. The magnetizing current is the current that flows through the stator windings to create the magnetic field. The back emf is the voltage that is induced in the stator windings by the rotating magnetic field.
Option A is incorrect because the supply voltage and the back emf are always in phase with each other in a synchronous motor.
Option B is incorrect because the magnetizing current is the current that flows through the stator windings to create the magnetic field. The back emf is the voltage that is induced in the stator windings by the rotating magnetic field.
Option D is incorrect because the torque angle is the angle between the rotating stator flux and the rotor poles.