The longest river plain in India is _______________.
A. Sutlej plain
B. Godavari plain
C. Krishna plain
D. Gangetic plain

Hint: The river with the longest plain in India is considered to be the most sacred river in the country. The longest river plain covers an area more than 12,500 square kilometres and is considered among the largest basins in the whole world.

Complete Answer:
Option A - The river Sutlej belongs to the five tributaries of the river Indus which gives Punjab its name. Sutlej is the longest of the five main tributaries that pass through the Indian state of Punjab. It originates in the Himalayas in southern Tibet. Its basin expands over an area of about 395,000 square kilometres.
Option B - The Godavari river originates in the Trimbakeshwar range situated in Maharashtra. It is the second-longest river after river Ganga and is also known as Dakshin Ganga. Its basin expands over an area of about 312,812 square kilometres.
Option C - The Krishna river is also known as Krishnaveni ranks fourth in terms of the river basin and water inflows, right after Ganga, Godavari and Brahmaputra rivers. Its basin expands over an area of about 258,948 square kilometres.
Option D - The Indo-Gangetic Plain additionally called the North Indian Plain lies between the Himalayas and the Deccan. The Ganges or Ganga River basin in India, mostly situated in Uttar Pradesh and Bihar states, frames the focal and head part of the plain. The Ganga basin covers more than 12,500 square kilometres in northern India and positions among the largest in the world in drainage basin and length. The whole Indo-Gangetic plain covers an area of about 2.5 million kilometres.

Thus, the correct answer is option (D).

Note: The general pitch of the plain is for all intents and purposes subtle, averaging just around 95 mm for each kilometre in the Ganges basin and marginally more along the Indus and Brahmaputra. This plain is of utmost importance because of the presence of the alluvial soils in this region and is also considered to be the largest alluvial plain in the world.