The correct answer is B. Theodore Hall
Explanation:
Theodore Hal was a scientist and more importantly a spy for the Soviet Union, indeed Hall participated in the development of atomic bombs during the Second World War and provide key information about this to the Soviet intelligence while he worked at Los Alamos National Laboratory in the U.S. Because of this the FBI led an investigation to uncover those that had participated in the espionage, but this was mainly attributed to Klaus Fuchs and others such as David Greenglas and Ethel and Julius Rosenberg that had also worked in the project and provided information to the Soviet Union but Theodore Hall was never charged and indeed Hall had a career and continue providing information to the Soviet Union after he left Los Alamos. Therefore, the spy that was never charged was Theodore Hall.
