Part A: The statement that BEST describes the theme of "Sophie's Challenge" is: A. Showing courage in the face of adversity helps overcome one's fear.
Part B: The piece of evidence that shows where the theme from Part A emerges in the text is: A. "'I'm not leaving Prim,' I said recklessly. 'I'm going to ride her to the pasture.'"
This statement captures a moment of courage in the face of potential danger or difficulty, aligning with the theme identified in Part A.