Which statement correctly contrasts AM, FM, and SSB in terms of spectral efficiency and typical uses?

• A. AM has simple demodulation and wide bandwidth but poor efficiency; FM offers better noise immunity with wider bandwidth; SSB is spectrally efficient (narrower) and used for long-range voice in HF.
• B. AM is the most efficient mode among the three.
• C. FM uses less bandwidth than AM.
• D. SSB is inferior for long-range HF voice due to high power requirements.

Answer: (A)

This question tests how spectral efficiency and typical uses differ among AM, FM, and SSB. AM transmits a carrier plus two sidebands, so demodulation is straightforward with a simple envelope detector. But power is wasted in the carrier and both sidebands, making AM relatively inefficient, and its spectrum is wide—about twice the modulating signal’s bandwidth. FM keeps the carrier amplitude constant and encodes information in frequency deviations, giving excellent noise immunity, but at the cost of a much wider occupied bandwidth. SSB, by removing the carrier and one sideband, is far more spectrally efficient and uses power more efficiently for the same audio quality, which is why it’s favored for long‑range voice on HF. The other choices misstate either the bandwidth relationships or typical uses: AM isn’t the most power-efficient, FM doesn’t use less bandwidth than AM, and SSB isn’t inferior for long-range HF voice.