Which statement best describes amplitude modulation?

The correct choice accurately captures a key aspect of amplitude modulation (AM). In amplitude modulation, the amplitude of the carrier wave is indeed varied in accordance with the modulating signal while the carrier frequency itself remains constant. This modulation results in sidebands being created, which are the additional frequencies that carry the information of the modulating signal.

In AM, the carrier wave serves as a constant frequency upon which the modulation occurs. As the amplitude of this carrier is varied, it produces changes in the sidebands’ amplitudes—these sidebands are the frequencies generated above and below the carrier frequency that contain the actual information being transmitted.

The other statements do not correctly represent amplitude modulation. Altering frequency in synchronization with a modulating signal pertains to frequency modulation, not amplitude modulation. The conversion of character data to audio tones is more aligned with methods of digital-to-analog conversion, rather than a description of AM. Lastly, while it mentions the change in amplitude, a choice that states only a single frequency being transmitted does not fully encompass the nature of AM, which involves both the carrier and the sidebands as an essential part of the transmitted signal.