Mericiful is a term used to call a person that shows mercy or practice mercy.While relentless is an action that shows harshness or inflexibility.Thus, these 2 words are antonyms since they are oppositely shows opposite meaning.When mercifyl is showing mercy , relentless is showing harsh things.Therefore the answer to this question is letter B. Antonyms.Antonym is a term used to call 2 opposite things, synonym is a term used to call the same meaning words and neither is neither antonym or synonym.