Antonyms are words that have contrasting, or opposite, meanings.
Choice C is correct. Relentless means 'harsh or grim' the opposite being Be merciful meaning 'forgiving'. For example: The sun burned down during the summer relentlessly. Choice A means 'merciless'. Choice B means 'a person who cannot rest or an uncomfortable feeling'. Choice D means 'rough'. Choice A, B, and D are wrong.