A book, by definition, is a collection of written, printed, or illustrated pages bound together. The one essential component of any book is its pages — without them, it cannot be called a book. While chapters, diagrams, and covers are common features in many books, they are not essential or universally present.
(A) Pages — Correct, every book must have pages.
(B) Chapter — Not all books are divided into chapters.
(C) Diagram — Not all books contain diagrams.
(D) Cover — Useful, but even a draft book can exist without a formal cover.