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)definite is correct as the word definite means clearly stated or decided; not vague or doubtful and the word amorphous means without a clearly defined shape or form.
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.
Nauseous, obscene, and providential means affected with nausea; feeling inclined to vomit, (of the portrayal or description of sexual matters) offensive or disgusting by accepted standards of morality and decency, and occurring at a favourable time; opportunerespectively.