- 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.
According to the rulings on this card's gatherer entry
01/10/2008 If the exiled card is an Aura, that card's owner chooses what it will enchant as it comes back onto the battlefield. An Aura put onto the battlefield this way doesn't target anything, but the Aura's enchant ability restricts what it can be attached to. If the Aura can't legally be attached to anything, it remains exiled forever.
It would appear that option 2 in your question is the correct interpretation.