What device is used to vary oscillator frequency?

The correct answer is a varicap. A varicap, or varactor diode, is a semiconductor device that acts as a voltage-controlled capacitor. By varying the voltage applied to the varicap, the capacitance changes, which in turn affects the resonant frequency of the oscillator circuit it is part of. This ability to control frequency makes varicaps particularly useful in applications like frequency modulation and in tuning circuits, where precise frequency adjustments are needed.

In contrast, amplifiers are primarily used to increase the power of a signal without directly adjusting its frequency. Receivers are designed to receive and process signals rather than to modulate or control their frequencies. Mixers combine two or more signals but do not inherently vary frequency like a varicap does. Hence, the varicap stands out as the device specifically tailored for varying oscillator frequency.