Which type of modulation changes the RF power output of a transmitter?

Amplitude modulation is the correct choice because it involves varying the amplitude of the carrier wave in accordance with the amplitude of the input signal. In essence, the power output of the transmitter changes with the modulation of the carrier wave; when the amplitude of the modulation increases, the power output increases as well. This is a fundamental aspect of amplitude modulation, making it unique compared to other modulation methods.

In contrast, frequency modulation and phase modulation maintain a constant amplitude of the carrier wave while varying either the frequency or the phase, respectively. These methods do not change the overall power output of the carrier signal; they simply encode information in different parameters of the wave. Digital modulation can involve various techniques, but it primarily focuses on transmitting information as discrete signals rather than altering the RF power output directly.