Old river channels and abandoned courses in Assam often form:

The correct answer is: b) Beels (wetlands).

Beels are depressions in the ground that are filled with water. They are formed when rivers change course or when the land subsides. Beels are important ecosystems that provide habitat for a variety of plants and animals. They also play a role in flood control and water purification.

Canyons are deep, narrow valleys that are carved out by rivers. They are formed when the river erodes the land over time. Canyons are often very beautiful and popular tourist destinations.

Sand dunes are hills of sand that are formed by the wind. They are found in

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