Which concept refers to varying the amplitude of a carrier signal?

The correct choice is Amplitude modulation, which specifically refers to the technique of varying the amplitude (the strength or intensity) of a carrier signal in relation to the information signal being transmitted. This modulation method allows the encoding of information onto a carrier wave, enabling the transmission of audio, video, or any other type of data over radio waves.

When the amplitude of the carrier signal changes in accordance with the amplitude of the signal carrying the information, this creates a variation that can be detected and demodulated by the receiving equipment. Consequently, the data can be recovered faithfully, making amplitude modulation a fundamental technique in broadcasting and communication systems.

In contrast to this method, frequency modulation involves varying the frequency of the carrier signal rather than the amplitude, whereas signal demodulation refers to the process of extracting the original information from a modulated carrier wave. Wave shaping typically applies to altering the waveform of signals rather than specific modulation techniques used for data transmission.