Which of the following types of modulation is used for data emissions below 30 MHz?

Explanation

Of the modulation types offered, frequency shift keying is the one that is authorized and in everyday use below 30 MHz: the keying shifts frequency, whether applied to the transmitter's own oscillator or produced by feeding audio tones into an SSB transmitter, and a constant-envelope frequency-keyed signal is narrow and tolerant of fading. Pulse modulation and spread spectrum are not authorized on HF at all, and DTMF tones on an FM signal are a signalling scheme rather than an HF data emission.