Based on what is historically known about Emperor Ashoka after the Battle of Kalinga, the best choice is:
c. the pillars proclaimed his regret at the bloodshed at the Battle of Kalinga and his conversion to Buddhism.
After the battle, Ashoka expressed remorse for the violence and suffering that resulted from the conflict. This led to his conversion to Buddhism and a focus on promoting peace, non-violence, and Dharma (moral law), which he conveyed through inscriptions on the pillars.