GMAT Sentence Correction (SC) | Re: Under a provision of the Constitution that was never

Under a provision of the Constitution that was never applied, Congress has been required to call a convention for considering possible amendments to the document when formally asked to do it by the legislatures of two-thirds of the states.

‘that’ refers to provision and its verb is ‘was never applied’.

Congress has been required
To call a convention for considering possible amendments to the document
When formally asked to do it

Present perfect ‘has been’ is used when a time of
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