Final answer:
Sadie's purpose for reading about multiple sclerosis is to gain understanding of her grandmother's condition. She seeks to comprehend the disease better through gathering information from books.
Step-by-step explanation:
When Sadie's grandmother is diagnosed with multiple sclerosis, Sadie decides to check out books from the library to learn more about the disease. The purpose behind Sadie's reading is A. to understand multiple sclerosis. This purpose is reflective of her desire to gain knowledge about her grandmother's condition, which implies seeking information to comprehend the intricacies and impacts of the disease more deeply.