Which type of modulation varies the strength of the transmitted signal?

The correct choice involves Amplitude Modulation, which is a technique where the strength, or amplitude, of the carrier wave is varied in accordance with the information signal being transmitted. In this type of modulation, the signal's amplitude changes while the frequency remains constant, allowing the audio or data to be represented by variations in signal strength. This property makes Amplitude Modulation effective for broadcasting audio signals, such as in AM radio.

Other modulation techniques work differently: Frequency Modulation varies the frequency of the carrier wave instead of its amplitude, Phase Modulation changes the phase of the wave, and Digital Modulation generally involves manipulating various aspects of a signal (including amplitude, frequency, and phase) to represent binary data. However, none of these techniques specifically describe modulation that solely varies the strength of the signal like Amplitude Modulation does.