The correct answer is D. no article.
A glossary is a list of terms and their definitions, usually found at the back of a book. It is not necessary to use an article before the word "glossary" because it is a general term that does not refer to a specific glossary.
The other options are incorrect because they are all articles. An article is a word that is used to modify a noun. In this case, the noun is "glossary." The articles "a" and "an" are used before singular nouns that begin with a consonant sound. The article "the" is used before singular nouns that are specific or unique. In this case, the glossary is not specific or unique, so it does not need an article.
