**Answer:**

The correct answer is option **D) her grandfather**.

Margie, the protagonist of the story "The Fun They Had" by Isaac Asimov, lived in a futuristic world where education was provided through a mechanical teacher, a computerized device. Margie had never seen a printed book before and was only familiar with electronic books displayed on a screen.

One day, Margie's grandfather found an actual book in his attic and showed it to Margie. He explained to her that in the past, people used to read books made of paper and ink, rather than using electronic devices like the mechanical teacher. This introduction to real books fascinated Margie and sparked her curiosity about how people in the past learned from them.

Margie's grandfather's knowledge and experience of the past made him the perfect person to tell her about real books. He had firsthand experience with printed books and could share his memories and insights with Margie. Moreover, as grandparents often play a role in passing down knowledge, traditions, and stories to younger generations, Margie's grandfather took on this role by introducing her to the concept of physical books.

By telling Margie about real books, her grandfather helped to develop her interest in discovering more about the past and how education used to be. This encounter with a physical book planted a seed in Margie's mind, making her realize that there was something unique and special about the way people learned in the past.

Overall, Margie's grandfather was the one who told her about real books and set her on a path of curiosity and exploration, questioning the efficiency and effectiveness of the mechanical teacher and electronic books.