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)unpleasant is correct as the word unpleasant means causing discomfort, unhappiness, or revulsion; disagreeableand the word palatable means (of an action or proposal) acceptable or satisfactory
Both the words mean opposite of each other.
The other options are not correct as they do not have the opposite meaning to the given word.