Which modulation technique is primarily used in amplitude modulation (AM) broadcasting?

Amplitude modulation (AM) is a technique used to encode information in a carrier wave by varying its amplitude. In AM broadcasting, the audio signal, which can be any form of sound, modulates the amplitude of a constant radio frequency carrier wave. This means that as the amplitude of the audio signal changes, the amplitude of the carrier wave is varied correspondingly, allowing the transmission of sound information over distances.

This modulation technique is specifically designed for broadcasting and is characterized by its simplicity and effectiveness in transmitting audio signals. The advantage of AM is that it can be received by simple receivers and is less affected by noise when compared to some other modulation techniques. It's commonly used in AM radio broadcasting for this reason, as it efficiently delivers audio content to a broad audience.

The other options are different modulation techniques that do not pertain directly to AM broadcasting: phase modulation involves changing the phase of the carrier wave, pulse modulation involves transmitting data by converting its amplitude and time characteristics into digital pulses, and frequency modulation varies the frequency of the carrier wave instead of its amplitude. Hence, amplitude modulation is the correct and most relevant choice for AM broadcasting.