The correct answer is: B. pull-in torque
Pull-in torque is the maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency. It is the minimum torque required to overcome the friction and windage losses of the motor and to accelerate the rotor to synchronous speed.
Pull-up torque is the torque that a synchronous motor can develop when it is already running at synchronous speed. It is greater than the pull-in torque because the motor is not accelerating at this point.
Pull-out torque is the maximum torque that a synchronous motor can develop without losing synchronism. It is greater than the pull-in torque because the motor is now running at synchronous speed and has a built-up magnetic field.
None of the above is the correct answer because it does not describe the pull-in torque.