Which of the following is a form of amplitude modulation?

Amplitude modulation (AM) is a method of varying the strength (amplitude) of a radio signal to encode information. Single sideband (SSB) is a refinement of AM where only one of the sidebands (the upper or lower) is transmitted, and the carrier wave is suppressed. This technique allows for more efficient use of bandwidth and power, making it a popular choice in amateur radio communications.

In SSB, the modulation still fundamentally relies on the concepts of amplitude modulation, as it modulates the amplitude of the signal to carry the audio or data information. SSB provides a clearer signal and extends the range of communication compared to traditional AM by reducing bandwidth usage and allowing for better signal-to-noise ratios.

The other options listed represent different types of modulation techniques or methods that do not inherently rely on amplitude modulation. Spread-spectrum techniques transmit information over a wider frequency range than the minimum bandwidth necessary, while packet radio uses discrete packets of data transmitted over radio frequencies. Phase shift keying relates to digital modulation techniques where the phase of the carrier signal is varied. Each of these methods operates on principles distinct from traditional amplitude modulation, reinforcing why single sideband stands out as that particular form of amplitude modulation.