Final answer:
The sentence that best explains the credibility of the claim made in the excerpt is: "A policy that results in the loss of 600 million people to the Communists, and a war which cost us 117,000 American casualties isn't good enough for America."
Step-by-step explanation:
The sentence from the excerpt that best explains the credibility of the claim is:
"A policy that results in the loss of 600 million people to the Communists, and a war which cost us 117,000 American casualties isn't good enough for America."
This sentence highlights the significant losses suffered by the United States and its allies in terms of human lives during the Truman-Acheson Administration. It suggests that the policy pursued by the administration has had grave consequences and should be reevaluated. The numbers stated in the sentence help to provide context and emphasize the severity of the situation.