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)profession is correct as the word profession means a paid occupation, especially one that involves prolonged training and a formal qualificationand the word pastime means an activity that someone does regularly for enjoyment rather than work; a hobby. 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.