The best way to rewrite the sentence while ensuring correct usage of standard written English is:
A: "Ever since the sting operation, there has been much opposition from those who maintain that it was an unauthorised act."
This option correctly uses "those" instead of "they," which maintains grammatical correctness in the context.
The correct option is (A): Ever since the sting operation, there has been much opposition from those who maintain that it was an unauthorised act.