The answer to the question "The river basin with the largest runoff in India is" is the Ganga basin (option D). Let's understand why this is the correct answer in detail:

1. Introduction to River Basins in India:
India is home to numerous rivers, and they are divided into various river basins. A river basin refers to the area drained by a river and its tributaries. It encompasses the entire land surface where the rainwater and other sources of water flow towards a common outlet, such as a river mouth or a lake.

2. Importance of Runoff:
Runoff refers to the portion of rainfall or other water sources that does not infiltrate the ground but instead flows over the surface. It plays a crucial role in replenishing water bodies, ensuring water availability for agriculture, and maintaining the ecological balance.

3. Ganga Basin:
The Ganga basin is the largest river basin in India and covers a significant part of the northern and eastern regions of the country. It extends over multiple states, including Uttarakhand, Uttar Pradesh, Bihar, Jharkhand, and West Bengal. The Ganga River, also known as the Ganges, is one of the most sacred rivers in Hinduism and holds immense cultural and religious significance.

4. Factors Contributing to Large Runoff in the Ganga Basin:
Several factors contribute to the Ganga basin having the largest runoff in India:

- Geographic Extent: The Ganga basin covers a vast geographical area, including the Himalayan region, where heavy precipitation occurs due to orographic rainfall. This results in a significant volume of water flowing into the river system.

- Glacial Melt: The Ganga basin is fed by numerous glaciers in the Himalayas. During the summer months, these glaciers melt, adding a substantial amount of water to the river system and increasing the runoff.

- Monsoon Rainfall: The Ganga basin receives a significant proportion of its annual rainfall during the monsoon season. The monsoon winds from the Bay of Bengal bring heavy rainfall to the region, resulting in increased runoff in the river.

- Tributaries: The Ganga basin is fed by several major tributaries, including the Yamuna, Son, Gandak, and Kosi rivers. These tributaries contribute additional water to the Ganga River, increasing the overall runoff.

- Human Activities: The Ganga basin is densely populated, and human activities such as agriculture, industrialization, and urbanization contribute to increased runoff through surface runoff from impermeable surfaces, drainage systems, and irrigation practices.

5. Conclusion:
In conclusion, the Ganga basin has the largest runoff in India due to its extensive geographic coverage, glacial melt, monsoon rainfall, significant tributaries, and human activities. Understanding the runoff patterns and characteristics of different river basins is essential for water resource management, flood control, and sustainable development in India.