The correct answer is D. shading coils.
Shading coils are used to produce a revolving field in a shaded pole single-phase motor. They are made of copper wire and are placed around the poles of the stator. When current flows through the shading coils, they create a magnetic field that opposes the main magnetic field. This causes the magnetic field to shift, creating a revolving field.
The other options are incorrect because they do not produce a revolving field. An inductor is a coil of wire that stores energy in a magnetic field. A capacitor is a device that stores energy in an electric field. A resistor is a device that resists the flow of current.
Shading coils are a simple and effective way to produce a revolving field in a shaded pole single-phase motor. They are used in a variety of applications, including fans, blowers, and pumps.