To determine which detail best supports the answer to Part A (presumably regarding the themes of intelligence testing and government control in "Examination Day" by Henry Slesar), we should consider what message or theme is highlighted in the answer to Part A.
The detail that most effectively supports the themes of intelligence being controlled and the consequences of being too intelligent in the context of a dystopian society is:
D. "'We regret to inform you that his intelligence quotient is above the Government regulation, according to Rule 84 Section 5 of the New Code.'" (Paragraph 56)
This detail specifically reveals the government's regulation concerning intelligence and highlights the serious implications of exceeding those limits, directly aligning with the story's central conflict concerning the examination and its consequences.