Which is the largest part of the northern plains and is formed of older alluvium?

Correct Option: Bhangar

The northern plains of India can be divided into four distinct regions based on relief features and alluvial deposits: Bhabar, Terai, Bhangar, and Khadar.

1. Bhangar:
This is the largest part of the northern plain and is formed of older alluvium. It lies above the floodplains of the rivers and presents a terrace-like feature. The soil in this region contains calcareous deposits locally known as kankar.

2. Khadar:
This region consists of newer, younger alluvial deposits formed by the rivers. It is renewed almost every year, making it extremely fertile and ideal for intensive agriculture.

3. Bhabar:
A narrow belt of pebble deposits lying parallel to the slopes of the Shiwaliks, where streams disappear.

4. Terai:
A wet, swampy, and marshy region located south of the Bhabar belt where streams re-emerge.

Therefore, the largest part of the northern plains formed of older alluvium is Bhangar.