What type of modulation involves changing the signal's amplitude?

Amplitude Modulation (AM) is the correct answer because it specifically involves varying the amplitude of a carrier signal in accordance with the information being sent. In this technique, the strength or height of the signal wave is altered to convey data, such as audio or video signals, while the frequency and phase remain constant.

This method is widely used in broadcasting, particularly in AM radio stations, where audio signals, such as music or spoken words, change the amplitude of a radio frequency signal to transmit the audio content over large distances. By manipulating the amplitude, AM allows receivers to demodulate the audio signal from the carrier wave effectively, enabling sound reproduction.

In contrast, the other modulation types focus on different aspects of the signal. Frequency Modulation (FM) changes the frequency of the carrier signal rather than its amplitude, while Phase Modulation (PM) alters the phase of the signal. Time Modulation, although less commonly referenced in basic modulation discussions, usually pertains to varying the timing of the signal. Thus, Amplitude Modulation stands out for its distinct method of conveying information through amplitude variations.