The correct option is
C Merciful
'Ruthless' is an adjective which means 'merciless, cruel, unforgiving';
Option C - 'Merciful' is an adjective which means 'forgiving, sympathetic, compassionate'. This is opposite of the meaning of the given word 'Ruthless'.
On the other hand, Option A - 'Majestic' is an adjective which means 'awesome, dignified, sublime', Option B - 'Pity' is an adjective which means 'sorrow, sadness, distress', Option D - 'Grief' is a noun which means 'pain, agony, distress'.
Therefore, the antonym of 'Ruthless' is Option C - Merciful.