'Thesaurus' (a book of synonyms and antonyms) is the correct spelling. Key spelling pattern: 'ae' comes after 'th', 'au' after the second 's', and 'rus' at the end. Think of it as 'ThE SAUrus' to remember. Options A (Thesarus), B (Thesaarus), and C (Thesaerus) all mix up the 'ae' and 'au' placement.