Which mahajanapada became the most powerful between the sixth and the fourth-centuries BCE?

Magadha

The correct answer is Option (3) → Magadha

"Between the sixth and the fourth centuries BCE, Magadha (in present-day Bihar) became the most powerful mahajanapada. Modern historians explain this development in a variety of ways: Magadha was a region where agriculture was especially productive. Besides, iron mines (in present-day Jharkhand) were accessible and provided resources for tools and weapons. Elephants, an important component of the army, were found in forests in the region. Also, the Ganga and its tributaries provided a means of cheap and convenient communication. However, early Buddhist and Jaina writers who wrote about Magadha attributed its power to the policies of individuals: ruthlessly ambitious kings of whom Bimbisara, Ajatasattu and Mahapadma Nanda are the best known, and their ministers, who helped implement their policies."