Antonyms are words that have contrasting, or opposite, meanings.
Choice A is correct. Grudging means 'unwillingly does something' the opposite being wholehearted meaning 'to give something willingly'. For example: The little girl gave her doll to the orphan wholeheartedly. Choice B means 'grudging. Choice C means capable of showing that something is right. choice D means 'to withhold an information'. Choice B, C and D are wrong.