The correct option is (a).
Explanation
The Himalayan drainage system comprises three major river networks: the Indus, the Ganga, and the Brahmaputra. These rivers are responsible for the formation of the Northern Plains and define the longitudinal extent of the Himalayan mountain chain.
Statement 1 is Correct
The Indus and the Brahmaputra are the major rivers of the Himalayan drainage system. They originate in the Tibetan Plateau near Lake Mansarovar. While their specific sources lie in the Trans-Himalayan region (north of the Great Himalayas), they are broadly categorized as Himalayan rivers because they cut through the Himalayan ranges, creating deep gorges, and are integral to the region's hydrology.
Statement 2 is Incorrect
The Northern Plains are not formed solely by the Ganga river system. They are formed by the interplay of three major river systems: the Indus, the Ganga, and the Brahmaputra, along with their tributaries. The plains are composed of alluvial soil deposited by these rivers over millions of years.
Statement 3 is Incorrect
The Brahmaputra marks the eastern-most boundary of the Himalayas, not the western-most. The Himalayas run in a west-east direction from the Indus (which marks the western boundary) to the Brahmaputra. Beyond the Dihang gorge, the Himalayas bend sharply to the south and spread along India's eastern border.
Key Takeaway
The Northern Plains are a depositional feature resulting from the Indus, Ganga, and Brahmaputra systems. The longitudinal extent of the Himalayas is defined by the Indus Gorge in the west and the Brahmaputra Gorge in the east.