I would say that the answer to the question would be A. Glossary. The glossary feature of a work traditionally appears at the end of a book and contains terms or vocabulary that has been featured or discussed in the work. It is a reference tool that allows for a quick cross check of items in the work.
The other options are not necessarily best equipped to answer the question. Diction, in this case, would be short for "dictionary" and that would be another reference work, standing entirely on its own. The Preface is something that appears at the start of a work. The other options simply do not contain the same information or meet the standard best as the term, "Glossary," does. With this in mind, I would think that "Glossary" best answers the question.