Which modulation technique varies amplitude rather than frequency?

The correct answer is Amplitude Modulation, as this technique specifically involves varying the amplitude of a carrier signal in order to encode information. In Amplitude Modulation (AM), the strength or amplitude of the carrier wave is changed in proportion to the waveform being sent, which typically corresponds to audio or voice signals. This change in amplitude allows the signal to carry the desired information over the communication medium, such as radio waves.

In contrast, Frequency Modulation (FM) varies the frequency of the carrier signal to convey information, while Phase Modulation (PM) alters the phase of the carrier wave. Digital modulation encompasses a range of techniques that may employ various approaches to encode data, but it does not specifically focus on amplitude variation. Thus, Amplitude Modulation stands out for its distinctive approach of modifying amplitude to transmit information effectively.