Final answer:
The sentence highlighting that Klaus Fuchs admitted to passing atomic bomb information to the Soviets best supports the claim against his participation in the Manhattan Project.
Step-by-step explanation:
The sentence that best supports the claim that Klaus Fuchs should not have been allowed to work on the Manhattan Project is: "In 1950, Klaus Fuchs, a British physicist, admitted to giving information about America's atomic bomb to the Soviets." This statement encapsulates the critical breach of security and trust, documentably proving that Fuchs was indeed a spy who passed on sensitive and classified information that significantly aided the Soviet Union in developing their own nuclear capabilities earlier than expected.
This admission not only underscores Fuchs' duplicity but also the gravity of the consequence—he enabled another nation to make a quantum leap in atomic technology, which could have posed a severe threat to global security during the post-World War II era.