Which of the following is a way to vary the capacitance of a variable capacitor?

Varying the capacitance of a variable capacitor can effectively be accomplished by changing the spacing between its plates. Capacitance is fundamentally defined as the ability of a system to store an electric charge, which depends on the surface area of the plates, the distance between them, and the dielectric material present between the plates.

When the distance between the plates is increased, the capacitance decreases because the capacitance is inversely proportional to the distance; thus, bringing the plates closer together increases the capacitance. This principle allows variable capacitors to tune the capacitance to fit specific applications, such as in tuning circuits.

The other options are not mechanisms for varying capacitance in a variable capacitor. Voltage affects the stored charge but does not intrinsically change the capacitor's capacitance. Changing the material of the plates can affect other properties like resistivity but doesn't directly impact capacitance in the same immediate way. Lastly, the frequency of the applied voltage relates more to the behavior and reactance of capacitors in an alternating current circuit rather than directly altering the capacitance itself.