A thesaurus is the correct answer. A thesaurus is a book that lists synonyms, or other words that mean the same thing. This will best help you find new ways to express your ideas. A dictionary simply defines a word, which could be somewhat helpful. A glossary is an alphabetized list found at the end of a book that includes new, uncommon, or specialized terms. Flashcards are a tool used to help study, so they would not be very helpful with learning new vocabulary.
