Find the part of the sentence that contains an error:
What renders the draft legislation particularly contentious is not its proposed realignment of fiscal powers per se, (1)/ but that it presumes, without empirical substantiation, (2)/ a fiscal equivalence among states whose developmental baselines are (3)/ incommensurable by any normative metric. (4)

Correct option is B

Option (b) contains an error.
Explanation: In the correlative construction “not X, but Y,” the elements linked by not … but … must be grammatically parallel.
Here, X is a noun phrase (“its proposed realignment…”), but Y is a that-clause (“that it presumes…”), breaking parallelism. Replace “but that it presumes” with a noun phrase such as “but its presumption (that …)” or “but the fact that it presumes ….”
Grammatical rule used: Parallelism with correlative conjunctions — items joined by not…but must match in form (NP↔NP, clause↔clause).
Example: “What worries me is not the delay ( NP), but the cost ( NP).”