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 D)willing is correct as the word willing means ready, eager, or prepared to do somethingand the word reluctant means of something provided or available) lacking in quantity or quality.unwilling and hesitant; disinclined.
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.