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 C)harshness is correct as the word harshness means the quality of being unpleasantly rough or jarring to the sensesand the word clemency means mercy; lenience.
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.