The correct answer is D) Frequency. The speed of a synchronous electric motor is directly proportional to the frequency of the alternating current (AC) supply. Increasing the frequency of the AC supply will result in an increase in the speed of the synchronous motor, as the motor's rotor will rotate faster to keep up with the changing magnetic field. The other options are incorrect because: A) Inductance affects the power factor of the motor, but does not directly impact the speed. B) Voltage changes can affect the torque of the motor, but do not directly influence the speed. C) Armature current changes can impact the torque, but not the speed, of a synchronous motor.