- Pacifism re-enters play enchanting the same creature as before
- Pacifism chooses a new target when it re-enters play
- Nothing -- Auras can only exist while enchanting a creature, so Pacifism disappears in a poof of smoke when it's exiled, similar to what tokens do.
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According to the rulings on this card's gatherer entry
It would appear that option 2 in your question is the correct interpretation.