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 B)pleasant is correct as the word pleasant means giving a sense of happy satisfaction or enjoymentand the word odious means extremely unpleasant; repulsive.
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.