I have several of the 1a variety, and have used 1d but didn't particularly like the whole derailleur system for use with an engine. I'd like to try it again with a real moped transmission that's built into the engine.
Around here you usually see 1a or 2d with an occasional 1d or 2c. 3c pretty much only exists in ebikes and everything else is uncommon except in fabricobbled machines, with the exception of 3a and 3b which I've only ever seen in off the shelf kits from italian or french manufacturers.