The correct answer is 1 and 2 only. Explanation & Key Takeaways: • Formed by the Indus, Ganga, and Brahmaputra rivers • The Northern Plains are alluvial plains created by the deposition of sediments (alluvium) brought by these major rivers. • They extend across Punjab, Haryana, Uttar Pradesh, Bihar, West Bengal, Assam, and parts of Pakistan and Bangladesh. • Prone to floods during the monsoon • Heavy monsoon rainfall and snowmelt in the Himalayas often cause rivers to overflow, leading to floods. • Fertile alluvial soil in the plains also results from flood deposits. • Made up mainly of basaltic lava • This is incorrect. Basaltic lava is typical of the Peninsular Plateau (e.g., Deccan Plateau), not the Northern Plains. • The Northern Plains consist of alluvium (clay, silt, sand, and gravel). Related Concepts: • Soil types: Bhangar (old alluvium) and Khadar (new alluvium). • Agricultural significance: Fertile soils support rice, wheat, sugarcane, and pulses. • Population: One of the densely populated regions of India due to fertile land and water availability. The Northern Plains are formed by Indus, Ganga, and Brahmaputra rivers and are flood-prone, but they are not made of basaltic lava.