To determine which detail best supports the answer to Part A, we need to consider the context of the question and what the answer to Part A was.
If Part A was focused on the social and political climate of fear and suspicion during McCarthyism, particularly regarding the threat of communism, then the best supporting detail would likely emphasize the general atmosphere of paranoia and conformity.
In that case, the most relevant choice would be:
A. "McCarthy capitalized on national paranoia by proclaiming that Communist spies were omnipresent and that he was America's only salvation." (Paragraph 6)
This detail directly indicates how McCarthy's actions played into the prevailing fear of communism and highlights the atmosphere of paranoia that characterized the era.
If the focus of Part A were on the consequences of this climate, then D could also be considered relevant, as it indicates how this fear manifested in societal behavior. However, based purely on the context of McCarthy's role in promoting fear, A is the strongest choice.