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)pertinent is correct as the word pertinent means relevant or applicable to a particular matter; appositeand the word desultory means lacking a plan, purpose, or enthusiasm
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.