The correct answer is: B. pull-out torque
The pull-out torque is the maximum value of torque that a synchronous motor can develop without losing its synchronism. It is the torque that the motor can develop at a speed slightly above the synchronous speed. If the load torque on the motor exceeds the pull-out torque, the motor will fall out of synchronism and start to run at a speed below the synchronous speed.
Slip torque is the torque that a synchronous motor develops when it is running at a speed below the synchronous speed. It is the torque that is required to overcome the friction and windage losses in the motor.
Breaking torque is the torque that a synchronous motor can develop when it is being braked. It is the torque that is required to stop the motor from rotating.
Synchronising torque is the torque that is required to bring a synchronous motor up to the synchronous speed. It is the torque that is required to overcome the inertia of the motor and the load.