The power developed by a synchronous motor will be maximum when the load angle is zero.
The load angle is the angle between the rotor and stator magnetic fields. When the load angle is zero, the two fields are aligned and the motor is operating at its maximum power.
When the load angle is greater than zero, the rotor field is lagging behind the stator field. This causes the motor to operate at a lower power.
When the load angle is greater than 90 degrees, the rotor field is leading the stator field. This causes the motor to operate at a negative power, which means that it is acting as a generator.
Therefore, the power developed by a synchronous motor will be maximum when the load angle is zero.