The northern plains of Bihar are characterized by numerous smaller rivers and streams that form a complex:

Delta system
Drainage network
Lake system
Wetland ecosystem

The correct answer is: b) Drainage network.

A drainage network is a system of rivers and streams that flow through an area and collect water from the land. The northern plains of Bihar are characterized by numerous smaller rivers and streams that form a complex drainage network. This drainage network is important for the region’s economy and environment. The rivers and streams provide water for irrigation, drinking, and industrial use. They also help to regulate the climate and prevent flooding.

The other options are incorrect because:

  • A delta system is a landform that is formed when a river meets the ocean. The northern plains of Bihar are not located near the ocean.
  • A lake system is a group of lakes that are connected to each other. The northern plains of Bihar do not have a large number of lakes.
  • A wetland ecosystem is an area of land that is saturated with water. The northern plains of Bihar are not a wetland ecosystem.