Daniel Ellsberg was primarily known for his role in the leaking of the Pentagon Papers, which revealed information about the United States' political and military involvement in Vietnam. He was not directly associated with the Nixon administration in terms of being an advisor or a member of any special projects like the "plumbers." Rather, he became a target of Nixon's political operatives after he leaked the Pentagon Papers, as they sought to discredit him and suppress further disclosures.
Therefore, the correct option is:
He was targeted by Nixon's political operatives.