Which of the following types of modulation is used for data emissions below 30 MHz?
Why On HF, data signals are almost universally sent by shifting the carrier between discrete frequencies, that is frequency shift keying. Classic RTTY uses direct FSK with a mark and space tone pair, and AFSK feeding an SSB transmitter produces the same on-air J2B/F1B type signal. FSK is favored below 30 MHz because constant amplitude tones survive fading and can be copied through noise better than amplitude-based schemes.
Watch out Spread spectrum is tempting because it is a modern data technique, but Part 97.305 only authorizes SS emissions on bands at 222 MHz and above; likewise pulse emissions are restricted to the microwave bands, and DTMF tones over FM are control/signaling tones, not a data emission.
HF data = shifting tones. Spread spectrum and pulse live up high (222 MHz and above), never below 30 MHz.