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)Gradually is correct as the word instantly means tat once; immediately and the word gradually means in a gradual way; slowly; by degrees. Both the words are opposite of each other.
The other options are not correct as repeatedly,lately,and awkwardly are not the opposites of the word instantly.