Destroy target red permanent, or return target mountain to owner's hand.
Many people are fools.
The decision to destroy a permanent or unsummon a Mountain is a decision made on announcement before a target of the proper type is selected. If the spell is redirected, this mode cannot be changed, so only targets of the selected type are valid. See Rule G.27 on Modal spells. This is essentially a Blue Elemental Blast that does not affect spells being cast. Instead of having the ability to counter red spells, it instead has the ability to Boomerang a Mountain .