Answer:
synonym
Step-by-step explanation:
Synonyms are words that mean basically the same thing. Plausible and reasonable both mean something that is believable, so they are synonyms. Boundless and infinite both mean the same thing as well, no limits and going forever. Bound means limited but boundless means not limited, so boundless and infinite are synonyms.
Antonyms are words that have opposite definitions. For example good and bad.
hope this helped :D