The correct answer is A. the synchronous speed.
A synchronous motor is an AC electric motor that runs at a constant speed that is synchronized with the frequency of the AC power supply. The speed of a synchronous motor is determined by the frequency of the AC power supply and the number of poles on the stator. The synchronous speed is given by the following equation:
$N_s = \frac{120f}{P}$
where $N_s$ is the synchronous speed in revolutions per minute (rpm), $f$ is the frequency of the AC power supply in hertz (Hz), and $P$ is the number of poles on the stator.
For example, a synchronous motor with 4 poles and connected to a 60 Hz power supply will have a synchronous speed of 1200 rpm.
The synchronous speed is the maximum speed at which a synchronous motor can run. If the motor is driven faster than the synchronous speed, the rotor will slip relative to the stator field. This slip causes the motor to develop a torque that opposes the driving force. As a result, the motor will not be able to run faster than the synchronous speed.
Options B and C are incorrect because a synchronous motor cannot run at less than or more than the synchronous speed. Option D is incorrect because it does not provide a correct answer.