Answer: b. As Thought Police they frame possible thought revolutionaries.
Step-by-step explanation:
Mr. Charrington and O’Brien are both revealed to be hidden members of the Thought Police, spying everyone around them, including Winston, to manipulate them and test their loyalty to the Party.
Mr. Charrington providing help for Winston’s several crimes are shown to be only a setup.
O’Brien´s participation in the revolutionary group called the Brotherhood is also a lie to trap Winston.