C. Appendix.
An appendix is a section at the end of a book that provides additional information or material about related topics that are not essential to understanding the central idea of the book. This information is often supplemental and can include data, graphs, charts, maps, or other reference material that supports or expands upon the main text.
The foreword is an introduction to the book written by someone other than the author, while the acknowledgements section typically includes the author's expressions of gratitude to those who helped with the book's creation. The index is a list of terms and topics with corresponding page numbers that allows readers to quickly locate specific information within the book.