Synonyms are words which have the same or nearly the same meaning in the same language.
Antonyms are words which have opposite meaning in comparison to the other.
Option A)retain is correct as the word retain means continue to have (something); keep possession ofand the word relinquish means voluntarily cease to keep or claim; give up.
Both the words are opposite of each other.
The other options are not correct as they do not have the opposite meaning to the given word.