The correct answer is D) a DC power supply. In a synchronous motor utilizing slip rings and brushes, the rotor is typically excited by a DC power supply. This DC excitation creates a constant magnetic field in the rotor, which interacts with the rotating magnetic field of the stator to produce torque and make the motor spin. The other options are incorrect because they do not provide the necessary DC excitation for the rotor. A step-up transformer (A) would only change the voltage level, not provide DC power. An AC power supply (B) would not be suitable for the DC-excited rotor. A half-wave rectifier (C) could provide DC, but would not be the most common or practical choice for this application.