The correct answer is: A. connected to D.C. supply
When a synchronous motor is started by induction motor action, the field winding is connected to a D.C. supply. This creates a magnetic field that causes the rotor to start spinning. Once the rotor is spinning at a speed close to the synchronous speed, the field winding is disconnected from the D.C. supply and the rotor is locked into synchronism with the stator field.
Option B is incorrect because short-circuiting the field winding would prevent the creation of a magnetic field and the motor would not start. Option C is incorrect because keeping the field winding open-circuited would also prevent the creation of a magnetic field and the motor would not start. Option D is incorrect because it does not include the correct answer.