DC voltage in the circuit varies the capacitance of which component to change the frequency of the oscillator?

The correct choice highlights the function of a varactor, which is a special type of diode designed to exploit the voltage-dependent capacitance inherent to its operation. In a circuit, applying a DC voltage to a varactor alters its capacitance, which in turn affects the resonant frequency of an oscillator circuit. This characteristic makes varactors particularly valuable in applications such as voltage-controlled oscillators (VCOs) and frequency modulation.

When the DC voltage is adjusted, it changes the depletion region in the varactor, leading to a variation in its capacitance; this allows for precise control of frequency without needing to change passive components in the circuit. As a result, this ability to tune frequency by simply modifying the applied voltage makes the varactor an essential component in many RF and analog systems. In contrast, resistors, transistors, and standard capacitors do not inherently provide this frequency-tuning capability based on DC voltage adjustments.