C. to look up synonyms for a word.
A thesaurus is a reference book or tool that lists synonyms and sometimes antonyms for words. Therefore, it would be most useful for a writer when they are searching for alternative words or expressions to avoid repeating the same word over and over again, to add variety and interest to their writing. It can help writers avoid overuse of certain words and make their writing more precise, colorful, and expressive. However, a thesaurus cannot provide information about the grammatical use of a word, spelling, or all possible meanings of a word.