How does phase modulation (PM) differ from frequency modulation (FM)?

Phase modulation (PM) differs from frequency modulation (FM) primarily in the way the information is encoded in the carrier signal. In phase modulation, the phase of the carrier signal is varied in accordance with the amplitude of the input signal, while the frequency of the carrier remains constant. This means that, after modulation, the carrier frequency does not change, allowing for a clear distinction in how phase and frequency are utilized in the modulation processes.

The other options do not accurately represent the fundamental differences between PM and FM. For example, FM does not inherently exhibit increased sensitivity to amplitude changes, nor does it modulate the amplitude of signals; FM modulates frequency changes. Additionally, FM generally requires greater bandwidth than PM due to the characteristics of its modulation process. Thus, the fact that the PM maintains a constant carrier frequency after modulation solidly distinguishes it from FM.