The best reference resource to help a reader determine the meaning of a technical word in a text is b) dictionary. A dictionary provides definitions, pronunciation, and usage of words, making it the most appropriate choice for understanding specific terms.
A thesaurus is useful for finding synonyms and antonyms, but it doesn’t provide definitions. An index is a list of topics or keywords in a book, and a glossary provides definitions of terms specific to a particular text or field, but a dictionary is the most comprehensive source for general meanings.