Final answer:
Option D, which states that Sinclair cared about poor working conditions, but his book led to changes in meat processing, is the piece of evidence that best supports the central idea from Part A.
Step-by-step explanation:
In Part A, the central idea is that Upton Sinclair's book, The Jungle, led to changes in meat processing. The piece of evidence that best supports this central idea is option D, which states that Sinclair cared about poor working conditions, but his book led to changes in meat processing.
Learn more about Upton Sinclair's book, The Jungle, and its impact on meat processing