Which was the first capital of Magadha?

Explanation:
Magadha was an old Indian domain that was arranged in the southern Bihar, and was viewed as one of the sixteen Mahajanapadas, in other words, 'Uncommon Kingdoms' of obsolete India.
Magadha accepted a critical part in the headway of Jainism and Buddhism, the Maurya Empire and Gupta Empire which are two of India's most significant areas, began in Magadha.
The city of Rajagriha is prominent in Jainism and Buddhism. Tirthankar Munisuvrata, the 20th jain expert was brought into the world here, and this spot is immovably associated with the arihant Mahavira and Gautama Buddha.
Both Lord Mahavira and Lord Buddha taught here during the 6th and the fifth century BC, Buddha was offered a forest house here by ruler Bimbisara. Moreover, the city of Rajgir became one of the Buddha's most huge educating regions.

Final Answer:

Hence, the correct option is C, Girivarja(Rajgriha).