Explanation: The circuit described is a classic example of a Phase-Locked Loop (PLL). A PLL works by comparing the phase (and thus frequency) of a voltage-controlled oscillator (VCO) to a stable reference frequency standard. Any difference detected results in an error voltage. This error voltage is then fed back to the VCO, typically adjusting a varactor diode's capacitance, which in turn precisely corrects the VCO's frequency to match the standard. This continuous feedback loop locks the VCO's output to the reference. A **Doubly balanced mixer** (A) is primarily used for frequency conversion or modulation, not for frequency stabilization of an oscillator. A **Differential voltage amplifier** (C) amplifies the difference between two voltages but is a generic amplifier and not the complete system described for frequency control. A **Variable frequency oscillator (VFO)** (D) is the component whose frequency is being controlled (the VCO itself is a type of VFO), not the entire control circuit.