Which of the following describes a common characteristic of variable capacitors?

Variable capacitors are designed to allow for the adjustment of capacitance values in a circuit. A common characteristic is that they often utilize air as their dielectric material. This is particularly true for many types of variable capacitors, like tuning capacitors used in radio frequency applications. The air dielectric provides benefits such as low loss, high stability, and good performance at high frequencies, which makes it ideal for applications where precise tuning is necessary.

In contrast to other options, using oil as a dielectric is not common for variable capacitors, as it generally pertains to specialized types of capacitors rather than variable ones. While variable capacitors adjust capacitance levels, they do not "fix" them, as that implies a lack of variability. The assertion that variable capacitors are exclusively used in power electronics is misleading, as they are more commonly found in applications like radio tuning and other frequency-sensitive environments rather than exclusively in power applications.