Part A
The main idea of "Mesmerized" is:
Dr. Mesmer relies on patients’ imaginations to make them believe in his power.
Part B
The key detail from the text that best supports the answer in Part A is:
“Dr. Mesmer said this force streamed from the stars and flowed into his wand. When he stared into his patients' eyes and waved the wand, things happened.”
This detail illustrates how Dr. Mesmer uses the concept of a mystical force to engage his patients' imaginations, which supports the main idea.