Answer : c. Neither
The words merciful and relentless are neither synonyms nor antonyms. They are not in any way related when their meanings are concerned. Merciful refers to the capability of showing mercy to others while relentless refers to being incessant or insistent. It is the manner of continuing without presenting a sign or hint of weakness. Synonyms refers to words that are of the same meaning while antonyms are those with opposite meanings. Merciful and relentless are neither of the same meaning nor opposing ones.