Which mode of transmission uses amplitude modulation?

The mode of transmission that uses amplitude modulation is specifically AM. Amplitude modulation is a technique used to encode information in a carrier wave by varying its amplitude, which directly corresponds to the information being transmitted, while the frequency remains constant. This modulation technique is commonly used in various broadcasting applications, most notably in AM radio.

In contrast, FM, or frequency modulation, varies the frequency of the carrier wave instead of the amplitude. SSB, or single sideband, is a refinement of amplitude modulation that eliminates one sideband and the carrier, allowing for more efficient transmission of the signal, thus it does not represent traditional amplitude modulation. PSK31 is a digital mode that uses phase shift keying to transmit data and does not utilize amplitude modulation at all. Therefore, the correct answer directly reflects the definition and characteristics of amplitude modulation.