Hmm, this is an easy one: b. Antonyms.
How would you know on the test though? Simple, I'll define the terms and give examples of each- you'll be a pro in no time!
a. Synonyms- words that mean the same thing. (happy, cheerful)
b. Antonyms- words that are opposites of each other. (happy; sad- also, malediction = curse, benediction = blessing)
c. Homophones- 2 words that sound alike but have different definitions. (deer, dear; their, there; night, knight)
d. Homonyms- 2 words that have the same sound/spelling, but mean different things. (Trunk = body of a tree, tree trunk; elephant nose, elephant trunk!)
Hope this helps :)