Explanation: A double-sideband (DSB) phone signal is characteristic of Amplitude Modulation (AM). Modulating the supply voltage to a Class C amplifier (or any RF amplifier) is a direct method to achieve AM. As the audio input varies the amplifier's supply voltage, the output RF power (and thus its amplitude) changes proportionally. This amplitude variation creates both an upper and a lower sideband around the carrier frequency, resulting in a DSB signal. Options A and B describe methods of producing Frequency Modulation (FM) by varying an oscillator's frequency. FM signals have constant amplitude. Option C describes a Phase-Locked Loop (PLL), primarily used for frequency control or demodulation, not for directly generating an amplitude-modulated DSB signal.