Which of the following is the main reason for the flatness of the Great Indian Plains?

The correct option is River deposition.

The Great Indian Plains, also known as the Indo-Gangetic Plains, are located between the Himalayan mountains in the north and the Peninsular Plateau in the south. These vast plains are primarily formed by the depositional work of three major river systems: the Indus, the Ganga, and the Brahmaputra, along with their numerous tributaries.

Over millions of years, these rivers flowing down from the Himalayas carried massive amounts of alluvium, silt, sand, and clay. As they reached the gentler slopes of the foothills and basin, their speed decreased, causing them to deposit these sediments in a deep depression (a foredeep or geosyncline). This continuous, long-term process of sedimentation filled the depression and gradually leveled out the terrain, resulting in an exceptionally flat and highly fertile alluvial plain.