The correct answer is ​The Brahmaputra River and its tributaries provided cheap and convenient means of communication.

• Magadha is the ancient kingdom of India and situated in west-central Bihar state, in northeastern India.
• It was the nucleus of several larger kingdoms or empires between the 6^th century BCE and the 8^th century CE.
• In ancient India from the 6^th century BCE onwards, the kingdom of Magadha made a mark for itself.
• Along with Bimbisara, the main Magadha actors who screamed war were his son Ajatashatru (492 BCE - 460 BCE) and the kings Shishunaga (c. late 5th century BCE) and Mahapadma Nanda (about mid-4th century BCE).
• The Magadhan armies under these aggressive rulers fought pretty much like any other kingdom's army in ancient India, using the prevalent four-fold army system (chariots, infantry, cavalry, and elephants), personally led by kings or princes.
• In many cases, intrigue was used as a means of war.
• The able royal leadership and the drive to expand territorially was the main reason for Magadhan success as it impacted heavily on its military system and modes of warfare.