In which application would you expect to use a variable capacitor?

A variable capacitor is specifically designed to allow its capacitance to be adjusted, which is essential in tuning applications, particularly in radio frequency circuits. In these circuits, the ability to change capacitance allows for fine-tuning of the resonant frequency of the circuit, which is crucial for selecting different radio stations and optimizing signal reception.

This functionality is central to many aspects of radio operation, such as in tuners and oscillators, making the use of a variable capacitor in tuning radio frequencies the most appropriate application. The other options do not typically align with the primary use case of variable capacitors. For instance, while capacitors are used in power supply circuits, they are usually fixed capacitors that do not require adjustment. Similarly, while small electronic devices may use various capacitors, the specific feature of being variable is not limited to them. High voltage applications often utilize fixed capacitors designed to handle specific voltage ranges rather than variable ones for tuning purposes.