The uniformitarian principle states that

a-landforms were created by fast changes

b-catastrophic events very likely changed the earths surface.

c-slow processes of erosion and deposition created landforms.

d-the earth was once completely covered with water

Option (C)

Explanation: According to the law of Uniformitarianism, the geological processes that is going on now, has undergone in the past and will be continued in the same way in the future too.

Due to this constant ongoing earth dynamic processes, in terms of millions of years, the previously exited landforms have undergone weathering and erosion, and subsequently deposition also has taken place, and gave the shape of the present existing landforms. These processes has occurred slowly and will be continued in the same way in the future also.

Thus, the correct answer is option (C).

a and b