The Erra Matti Dibbalu (Red Sand Dunes) near Visakhapatnam are a distinctive geological feature formed by:

The correct answer is: a) Wind erosion.

The Erra Matti Dibbalu (Red Sand Dunes) near Visakhapatnam are a distinctive geological feature formed by wind erosion. The sand dunes are made up of red sandstone, which is a type of sedimentary rock that is formed from the erosion of other rocks. The sandstone is then transported by the wind and deposited in the area where the dunes are located. The wind continues to erode the sandstone, which causes the dunes to change shape over time.

The other options are incorrect because:

• Volcanic activity is not the main cause of the Erra Matti Dibbalu. The sand dunes are not made up of volcanic rock.
• River deposition is not the main cause of the Erra Matti Dibbalu. The sand dunes are not located near a river.
• Glacial movement is not the main cause of the Erra Matti Dibbalu. The sand dunes are not located in an area that has been affected by glaciation.