Fa Hien visited India during the reign of

Concept: A pilgrim's travel account can be dated by cross-referencing the ruler he names or describes against the independently established chronology of Indian dynasties (inscriptions, coins, and dated grants).

Application: Fa Hien (Faxian), a Chinese Buddhist monk, travelled through India between 399 CE and 412 CE and recorded a prosperous, well-administered kingdom in his account, "A Record of Buddhistic Kingdoms." This period falls within the reign of Chandragupta II (Vikramaditya) of the Gupta dynasty, who ruled from approximately 380 CE to 415 CE -- the two timeframes overlap directly.

Cross-check (contrast the other rulers offered):

• Harshavardhana ruled from 606 CE to 647 CE, about two centuries after Fa Hien's visit; the Chinese pilgrim associated with Harshavardhana's reign is Hiuen Tsang (Xuanzang), not Fa Hien.

• Ashoka ruled from about 268 BCE to 232 BCE, nearly six centuries before Fa Hien travelled to India, so the Mauryan period is ruled out entirely.

Result: Fa Hien's own dated itinerary places his visit squarely inside Chandragupta II's reign, confirming Chandragupta II as the ruler during whose reign Fa Hien visited India.