The maximum constant load torque under which a synchronous motor will pull into synchronism at rated rotor supply voltage and frequency is known as

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.