Which of the following would most likely be formed by erosion? mountain, valley, flat plain, volcano.

The answer is Mountains. Erosions reduce the mass on mountain ranges, but by eroding, the tectonic plates beneath mountains accelerate their movements along the crust into the heart of mountain ranges which help give the mountain new form and height. Erosion in this sense, creates pressure below the mountain that shapes the mountain internally through altering tectonic plates. The Appalachian mountain is one such example as it has been formed by faults that were affected by erosions.

Answer they are produced by collision of tectonic plates;