The correct answer is D. zero.
Speed regulation is the change in speed of a motor as the load on the motor changes. In a synchronous motor, the speed is constant and does not change with load. This is because the speed of a synchronous motor is determined by the frequency of the power supply and the number of poles on the motor.
Option A is incorrect because 1% is not a typical value for speed regulation of a synchronous motor.
Option B is incorrect because 0.5% is not a typical value for speed regulation of a synchronous motor.
Option C is incorrect because speed regulation is not always positive. In a synchronous motor, the speed regulation is always zero.