You want a summary of the book?
Have you ever watched a dull movie? Have you been bored watching the teacher present slides? Well imagine the teacher presenting every single slide, and each slide has over 1000+ words. Yeah, you would be bored. This is why using the main idea in a presentation is needed; it prevents long slides that could take hours to finish, but it shortens the slide to a point where it can be easily understood without unnecessary detail.
Ponder means "to think about (something) carefully," which is most similar to D. Reflect. Ponder basically means, "to reflect on" something.