Final answer:
Boldface type words followed by definitions in the margins of a textbook indicate a glossary (option a), which is a collection of definitions for key terms. The glossary may be found at the end of a chapter or in an appendix of the book, assisting students in understanding the material.
Step-by-step explanation:
The terms in boldface type that are followed by a definition in the margins of a textbook refer to a glossary. A glossary is a section, often found at the end of a chapter or book, that provides the meanings of key terms used throughout the text. This feature allows students to quickly find and understand important concepts. In some texts, the glossary can also be an appendix, which is an additional section at the end of the book. The appendix may include various types of supplementary information, such as a glossary of key symbols and notations, in addition to other resources like a bibliography, photo credits, or lists of organizations and texts to know.
It's important for students to know how to navigate these parts of a textbook to enhance their understanding of the material. The glossary enables efficient learning by allowing students to reference the meanings of key terms as needed. Moreover, words from the glossary are sometimes hyperlinked in the digital versions of textbooks, making it even easier to access their definitions.