Answer: C- synonyms and antonyms
Explanation: It's the type of book (or online resource, just look it up) that has every work with the same or similar meaning to the one you look up (snyonym) and words with the exact opposite meaning (antonym) although sometimes the books just contain the synonyms.
For example, if I searched "happy" I'd get words like "cheerful" "ecstatic" "jubilant" and for antonyms I'd get "sad" and any synonyms of sad like "depressed" or "miserable"