The correct option is D Magadh
The most powerful city state of India in the 6th century BC was Magadh. The Magadha Empire came into existence when the four Mahajanpada- Magadha, Kosala, Vatsa and Avanti were engaged in the struggle for the supremacy from the 6th Century BC to 4th Century BC. Ultimately, Magadha emerged as the most powerful and prosperous kingdom in the North India.