Why did Asoka build stone pillars across his empire?

All the pillars of Ashoka were built at Buddhist monasteries, many important sites from the life of the Buddha and places of pilgrimage. Some of the columns carry inscriptions addressed to the monks and nuns. Some were erected to commemorate visits by Ashoka.

Explanation:

answer; (d) one tectonic plate plunges beneath another; (most common,convergent boundaries)

a. the moon was locked into orbit by the earths gravity as it was passing by and capture hypothesis i think