The correct answer is C. After the Kalinga War, Ashoka was moved by the violence, slaughter and sufferings to the combatants and noncombatants in the war. He decided to never wage any war again and adopted Buddhism, which is opposed to violence.
Option A is incorrect because the political unity of Mauryan India had already been achieved before the Kalinga War.
Option B is incorrect because Ashoka did not feel apologetic about the destruction of men and material on both sides in the war. He was moved by the violence, slaughter and sufferings to the combatants and noncombatants in the war.
Option D is incorrect because Ashoka adopted Buddhism shortly after the Kalinga War, not before it.