A synchronous motor develops maximum power when load angle is

The other options are incorrect because they do not correspond to the maximum power point of a synchronous motor. When the load angle is 0°, the torque developed by the motor is zero. When the load angle is 90°, the torque developed by the motor is also zero. When the load angle is 120°, the torque developed by the motor is negative. Therefore, the synchronous motor does not develop maximum power when the load angle is 0°, 90°, or 120°.