Explanation

Since the final part of the sentence is meant to go against the idea that the Second Amendment provides a “guarantee” of a private right, you need to fill the blank in the first part with something that reinforces the guarantee. Ironclad means “rigid or fixed,” and does what you need here. Defeasible and questionable would
undermine the guarantee. Nothing in the passage has any bearing on whether the guarantee is earnest.

An ironclad.
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