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 A)pleasing is correct as the word insipid means lacking flavour; weak or tastelessand the word pleasing means satisfying or appealing. 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.