The correct option is A Son
Patliputra, modern day Patna was a city built by Ajatashatru in 490 BCE as a small fort near the river Ganges. Its central location in northeastern India led rulers of successive dynasties to base their administrative capital here, from the Nandas, Mauryans, Shungas and the Guptas down to the Palas. It is Situated at the confluence of the Ganges, Gandhaka, and Son rivers, Pataliputra formed a "water fort or jaldurga".