Explanation:

- Ashoka: Known for his role in spreading Buddhism, Ashoka had rock edicts and stupas, but there's no historical record of him holding a religious assembly at Prayag every five years.

- Kanishka: A prominent Kushan emperor known for his patronage of Buddhism, particularly the Fourth Buddhist Council in Kashmir. However, no tradition of five-year assemblies at Prayag.

- Harshavardhana: Harshavardhana did indeed hold a religious assembly at Prayag every five years. He was a patron of Buddhism and encouraged religious debates and assemblies.

- Chandragupta Vikramaditya: Known for his patronage of the arts and sciences during the Gupta Empire but not specifically linked to religious assemblies at Prayag.