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

Below 30 MHz (in the HF bands), Frequency Shift Keying (FSK) is the standard modulation type used for data emissions like RTTY and various digital modes. In FSK, the carrier frequency is shifted between two or more specific frequencies to represent digital ones and zeros (mark and space). While Phase Shift Keying (PSK) is also common, FSK is the fundamental choice listed in the options. Audio Frequency Shift Keying (AFSK) is often used to generate the signal, which results in FSK over the air. Distractors like pulse modulation and spread spectrum are generally restricted or prohibited on most HF bands due to their wide bandwidths. DTMF over FM is typically used for VHF/UHF voice repeater control, not HF data.