If you return an Aura card to the battlefield this way, the Aura isn't being cast, so you choose what it will enchant as is enters the battlefield. An Aura returned this way doesn't target anything (so it could be attached to an opponent's permanent with hexproof, for example), 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 in the graveyard.

An Aura returning to the battlefield this way can't be attached to anything else entering the battlefield at the same time. If it enchants a permanent, you have to choose one already on the battlefield.