Step 1: Spot the error. The phrase "from they who maintain" in option (a) is grammatically wrong. "They" cannot function as the object of the preposition "from."
Step 2: Analyze options.
- (a) Wrong → "they who" is incorrect.
- (b) Correct → "those who" is the proper usage.
- (c) Wrong → "they who" again is incorrect, and "had been" unnecessarily changes the tense.
- (d) Acceptable → "those maintaining" is grammatically fine, but less formal and less precise than (b).
Step 3: Final correction.
The best and most standard version is option (b).
\[
\boxed{\text{Ever since the sting operation, there has been much opposition from those who maintain …}}
\]