Which modulation technique involves varying the phase of the carrier signal?

Phase Shift Keying (PSK) is a modulation technique that specifically alters the phase of a carrier signal to represent data. In PSK, different phases of the carrier wave correspond to different symbols or bits of information. This phase variation allows for greater resistance to noise and other interferences compared to amplitude modulation techniques.

In contrast, the other options involve different methods of modulation. Amplitude Modulation (AM) varies the amplitude of the carrier signal, while Frequency Modulation (FM) alters the frequency instead of the phase. Pulse Width Modulation (PWM) modifies the width of the pulses in the signal, which also does not involve phase changes. Therefore, PSK stands out as the choice that specifically varies the phase of the carrier signal, making it the correct answer to the question.