The erosional activity of rivers has contributed to the formation of:

Pediplains
Inselbergs
Volcanic plateaus
Ghats

The correct answer is A) Pediplains.

A pediplain is a gently sloping erosional surface that forms when a river system has eroded a landscape for a long period of time. The river will erode the land at a faster rate in the areas where the land is higher, and at a slower rate in the areas where the land is lower. This causes the land to become more and more level over time, until it forms a pediplain.

B) Inselbergs are isolated hills or mountains that rise above a surrounding plain. They are formed when the surrounding plain is eroded away, but the inselberg is protected by a harder rock layer.

C) Volcanic plateaus are large, flat areas of land that are formed by volcanic activity. They are typically made up of basalt, which is a type of lava that is very hard and resistant to erosion.

D) Ghats are a series of mountain ranges that run along the western coast of India. They are formed by the collision of the Indian and Eurasian tectonic plates.