To the best of my knowledge, it would be the first option, "Arcane/Elemental/Spell/Weapon Stun Limb"; in the case of the full incant, I'm fairly sure it should be, "With Eldritch Force I Stun your Limb".
As a side note, the Stun Limb description specifically notes that NPCs playing creatures without arms or legs should still accept the effect and roleplay it in a similar way, implying that they should simply apply the effect to the closest approximation of the specified limb if they have one (Stun Arm shouldn't work on something like a Horse that has only legs, but would probably work on something like a Harpy that has wings).