The correct option is A Kind
The word 'ruthless' refers to showing no pity or compassion towards others.
Option A, kind refers to the quality of being friendly, generous and considerate.
Option B, soft refers to a sympathetic, considerate and lenient attitude.
Option C, forgetful refers to being absent-minded and likely not to remember.
Option D, brutal refers to being unpleasant or harsh.
Thus, according to the meaning, the word 'kind' is the farthest in meaning from the word 'ruthless'.