1. The relation is part to whole.
2. A chapter is part of a book.
3. A page is a smaller unit, not the whole.

Answer: Book.

Keep the direction consistent: part first, whole second. Reversing it would make 'page' look plausible.

Shortcut. A petal is a part of the flower, so look for the whole a chapter forms: 'Book'. 'Page' is a smaller unit and 'Author' is not a whole.

Common trap. "Story" — a chapter does advance a story, but a story needs no chapters; the chapter is a structural division of a book, as a petal is of a flower.