Correct Answer: Option A) 1 & 2 only
Statement 1 is correct: The Brahmaputra River indeed enters the plains of Assam after flowing through a narrow gorge known as Dihang or Siang. After originating in Tibet (where it's known as Tsangpo), it flows through the Himalayan ranges and enters India through Arunachal Pradesh, where it passes through this narrow gorge before entering the plains of Assam.
Statement 2 is correct: Majuli Island, located in the Brahmaputra River in Assam, has historically grown in size due to alluvial deposition from the river. It's one of the largest riverine islands in the world, formed by the deposition of silt carried by the Brahmaputra.
Statement 3 is incorrect: The Brahmaputra River system is not confined only to Northeast India. The river originates in the Angsi Glacier in the Himalayas in Tibet, where it's known as Yarlung Tsangpo. It flows through Tibet, enters India through Arunachal Pradesh, flows through Assam, and then enters Bangladesh (where it's called Jamuna) before merging with the Ganges and emptying into the Bay of Bengal. Therefore, the river system extends beyond Northeast India to Tibet and Bangladesh.
