Which type of modulation is most commonly used for VHF and UHF voice repeaters?
The answer is D. FM, or Frequency Modulation. I’m not sure there is a technical reason for this, as any modulation would work just as well. Except PSK which really isn’t a modulation, but a digital mode, and you can use it in FM or SSB just as easily.
The reason probably lies more in a bit of circular effect. VHF/UHF radios get built with FM capabilities, and therefore the repeaters get built with the same, and repeat. FM is of course used in commercial applications in this range also, so adapting manufacturing for amateur radio products was probably a short move for producers.
FM also uses more bandwidth than SSB, 25kHz vs. 3kHz. In upper 10m and beyond, this bandwidth becomes more and more usable.
SSB can still be found in the VHF/UHF range, though! AM, not so much, but aircraft and airports do use AM in the 108-137 MHz range, and military aircraft are known to use AM from 225-400 MHz.