Explanation: An FM-phone signal is produced by varying the *frequency* of a radio wave in proportion to the modulating audio signal. **D) By feeding the audio directly to the oscillator.** This is the most direct method for generating Frequency Modulation (FM). When the audio signal directly influences the frequency-determining components of an oscillator (e.g., through a varactor diode or by altering the reactance of a tank circuit), it causes the oscillator's output frequency to deviate in sync with the audio. This direct frequency deviation is the essence of FM. **A) By modulating the supply voltage to a class-B amplifier.** **B) By modulating the supply voltage to a class-C amplifier.** Modulating the supply voltage of a power amplifier (Class B or C) causes its *output amplitude* to vary in proportion to the audio. This method produces Amplitude Modulation (AM), not Frequency Modulation (FM). **C) By using a balanced modulator.** A balanced modulator is used to generate a Double Sideband Suppressed Carrier (DSB-SC) signal. This is a crucial step in producing Single Sideband (SSB) signals, which are a form of amplitude modulation where the carrier and one sideband are removed. It does not produce FM.