Which of the following is a form of amplitude modulation?

Single sideband (SSB) is indeed a form of amplitude modulation, specifically a refinement of traditional amplitude modulation. In SSB, instead of transmitting both the upper and lower sidebands along with the carrier wave as in conventional amplitude modulation, only one of the sidebands is transmitted. This effectively reduces the bandwidth required and enhances efficiency, allowing for better utilization of the spectrum. By eliminating the carrier and one of the sidebands, SSB signals consume less power and provide improved signal clarity over longer distances.

Other forms like spread spectrum, packet radio, and phase shift keying (PSK) serve different purposes and use distinct techniques. Spread spectrum techniques involve spreading a signal over a wider frequency range to minimize interference and improve security. Packet radio is a method of digital communication that sends data in packets rather than continuously, and PSK is a form of digital modulation that encodes information in the phase of a carrier wave, rather than its amplitude. While these methods are significant in their own right, they do not represent amplitude modulation.