The best explanation of the events that led to the spread of Buddhism during Asoka's reign is:
Asoka took over Kalinga by force. Asoka rose to power through civil war. Asoka adopted Buddhism. Asoka became a just and compassionate ruler.
This option effectively outlines the sequence of Asoka's rise to power, the pivotal event of the Kalinga War that led to his adoption of Buddhism, and his transformation into a ruler who promoted just and compassionate governance, which ultimately contributed to the spread of Buddhism.