Concept: A landform with a steep slope on one side and a gentle slope on the other is called an escarpment.Explanation:• A ridge has high ground with slopes descending on both sides.• A knoll is a small, rounded hill.• A plateau is a flat, elevated area of land.• An escarpment is a hill or long cliff with a steep face on one side and a gentler slope on the other.Answer: D. an escarpment