Because the land is highly productive for cultivation.
Explanation
The Indo-Gangetic plains are vast alluvial plains formed by the depositional work of three major river systems: the Indus, the Ganga, and the Brahmaputra. These plains represent one of the most extensive and fertile continuous tracts of land in the world.
Analysis of Options
- Because of the presence of gold mines. is Incorrect: The Indo-Gangetic plains consist of deep alluvial deposits. Mineral resources, such as gold mines, are typically associated with the igneous and metamorphic rocks found in the Peninsular Plateau region (e.g., Kolar in Karnataka), not in alluvial plains.
- Because the land is highly productive for cultivation. is Correct: The plains are composed of rich alluvial soil brought down by rivers, which is exceptionally fertile. Combined with a flat terrain that facilitates mechanization and irrigation, and the availability of perennial water, this region is highly productive for cultivation. This agricultural abundance supports a massive population base.
- Because of the presence of steep slopes. is Incorrect: The plains are characterized by a homogeneous, flat, and level topography with a very gentle gradient. Steep slopes are a feature of mountainous regions (like the Himalayas), which generally restrict habitation and infrastructure development.
- Because of the cool climate similar to mountains. is Incorrect: The climate of the Indo-Gangetic plains is continental to subtropical, characterized by hot summers and cold winters, unlike the consistently cool and pleasant climate found in high-altitude mountain regions.
Key Takeaway
The high population density of the Indo-Gangetic plains is primarily driven by the fertile alluvial soil and flat terrain, which sustain intensive agriculture and facilitate the development of transport and settlement infrastructure.