slobster
Hero
To me it's the least believable, in that you can pick up a class - Rangering, Thieving, Magic Use, Clericism, Bard, any of which in theory should take years of study and practice to even get to 0th-level in never mind 1st - on a whim!
To me that issue is easily solved, for those to whom it matters, by requiring that the PC take some in-game time off to train with a mentor of the appropriate class. That might require some money, or another PC or friendly NPC may be willing to do it pro bono. May involve some RP at the table. It's in the GM's hands.
Bingo, done, and anyone for whom this isn't an issue can simply mark their new level of sorcerer down and continue the game.