Which type of modulation represents a change in the signal amplitude only?

The type of modulation that represents a change in signal amplitude only is Amplitude Modulation. In Amplitude Modulation (AM), the amplitude (the height of the wave) of the carrier wave is varied in proportion to the information signal being sent. This means that while the frequency and phase of the carrier wave remain constant, it is the amplitude that changes to encode the information.

AM is commonly used in radio broadcasting, where changing the amplitude of the signal corresponds to the audio information (like music or voice) being transmitted. Because this method relies solely on varying amplitude, it affects how the signal is interpreted but does not alter its frequency or phase characteristics.

In contrast, Frequency Modulation changes the frequency of the carrier wave to represent the information, while Phase Modulation modifies the phase of the wave. Pulse Width Modulation, on the other hand, adjusts the duration of pulses in a pulse train rather than the amplitude of a continuous wave. Hence, Amplitude Modulation distinctly focuses on changes in amplitude alone.