Explanation:

The earliest capital of the powerful Magadha Kingdom in ancient India was Rajagriha (modern Rajgir) (Option B). This strategic location, surrounded by five hills, provided natural defenses. Later, during the reign of Udayin, the capital was shifted to Pataliputra (Option A), which became a prominent imperial city. Vaishali (Option C) was the capital of the Vajjian confederacy, and Varanasi (Option D) was a significant city but not the earliest capital of Magadha.