Which city served as the capital of the ancient kingdom of Magadha during the early Vedic period?

The correct answer is ​Rajagriha.

Key Points

•  The capital of Magadha was Girivraj (modern Rajgir, Bihar). This city was sorrounded by rings of mountains and was, therefore, difficult to lay siege.
• The capital of Magadha was shifted from Rajagriha to Pataliputra by King Udayin.

Additional Information

•  In Ancient history, in the 6th century BC, North Indian provinces were divided into seven kingdoms out of which Avanti, Kosla, Vatsa, and Magadha.
• Among which Magadha rose as an emerging power defeating other surrounding provinces.
• Also, Magadha was a good centre for trade and fertile land. 
• The first dynasty ruled over Magadha was the Haryanaka Dynasty whose prominent rulers were Bimbisara and Ajatshatru.
• Bimbisara made Rajagriha as the capital.