212.4j.

If an Aura is coming into play under a player's control by any means other than by being played, and the effect putting it into play doesn't specify the object or player the Aura will enchant, that player chooses what it will enchant as the Aura comes into play. The player must choose a legal object or player according to the Aura's enchant ability and any other applicable effects. If the player can't make a legal choice, the Aura remains in its current zone, unless that zone is the stack. In that case, the Aura is put into its owner's graveyard instead of coming into play.