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)encourage is correct as the word encourage means give support, confidence, or hope to (someone)and the word curb means a check or restraint on something.
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.