The correct answer is "Synonyms"
Step-by-step explanation:
A thesaurus is a type of dictionary or document that lists the words of a language, but different to regular dictionaries thesaurus does not provide the definition of words but the synonyms or related words of a word or concept and can even include the antonyms of word or words that belong to the same field. This type of dictionaries is useful for writing and reading as using them help the reader to understand a word or a writer to avoid the unnecessary repetition of words. Considering this, it can be concluded a thesaurus is mainly a dictionary of "synonyms" as it includes synonyms or related words of a language.