The correct option is Tarai.
Explanation
The Northern Plains of India are divided into four distinct zones based on variations in relief features and the depositional characteristics of rivers flowing down from the Himalayas. From north to south, these zones are the Bhabar, Tarai, Bhangar, and Khadar.
Detailed Analysis:
- Bhabar: This is a narrow belt ranging between 8 to 10 km parallel to the slopes of the Shiwaliks. It consists of pebble and rock debris deposited by rivers. Due to the high porosity of the rocks, streams disappear in this belt. It is not marshy.
- Tarai: Located south of the Bhabar belt, this is the zone where the streams and rivers that disappeared in the Bhabar region re-emerge. The re-emergence of water creates a wet, swampy, and marshy region known as the Tarai. Historically, this zone was covered with thick forests and supported a rich variety of wildlife, although large areas have now been cleared for agriculture.
- Bhangar: This represents the largest part of the Northern Plains and is formed of older alluvium. It lies above the flood plains of the rivers and presents a terrace-like feature. The soil in this region contains calcareous deposits locally known as 'Kankar'.
- Khadar: This zone consists of newer, younger deposits of the floodplains. The soil is renewed almost every year, making it highly fertile and suitable for intensive agriculture.
Key Takeaway:
The Tarai is the specific physiographic zone characterized by the re-emergence of underground streams, resulting in marshy and swampy conditions with dense natural vegetation.