Why are landslides considered erosion?

Landslide also called as Landslip, is physical displacement of geomaterials, it is natural / man induced and causes removal or erosion of the part of land.

Explanation:

A landslide is a form of mass wasting including a wide range of ground movements including debris fall, slope failure, and mudflows.  Some landslides occur under oceans called as submarine landslides. Gravity is a prime driving force, and some are associated with earthquakes and rainfall. Physical and chemical weathering also takes place as a result of the landslides as it erodes the surface of the object whatever it comes in contact with.  Landslides caused by humans have led to Deforestation, blasts in mining and vibrations from machinery, and habitat loss. Landslides occurring at greater depths have the potential to trigger large Tsunamis and destroy natural harbors. Landslides characterized by debris flows are full of mud, soil rocks that move with great speed and are often hazardous.

The answer to the question is c.