I am confused as to why (A) is correct. Why should we assume that parties to a contract are agreeing that states DO have inherent powers to unilaterally change contracts? The parties could be explicitly stating in the contract that states DO NOT have those inherent powers. If so, such an explicit statement would not weaken the argument at all. For it could just be an additional provision in the contracts, complementing termination and modification clauses.
In my opinion, (D) would weaken the argument more because it posits that modification clauses, even if contained in agreements, can be countered by general principles of law that pose some conditions related to financial balance. Thus, a government's capacity to modify such agreements would not depend solely on the contract clauses established by the parties.
What am I missing here?