What is a delta?

A delta is indeed a landform that is created by the accumulation of sediment where a river meets a larger body of water, such as an ocean or a lake. As the river flows, it carries sediment that gets deposited when the water slows down upon meeting the larger body of water. This process can lead to the formation of a fan-shaped area, often characterized by rich soil and diverse ecosystems, making deltas important for both ecological health and human agriculture.

The other choices describe different geological features or processes but do not accurately define a delta. For example, mountains formed by erosion pertain to a process shaped primarily by physical weathering and does not involve sediment deposits from river systems. A flat plain with deep soil layers suggests a specific landscape type without the dynamic sedimentary processes characteristic of a delta. Similarly, a tributary refers to a smaller river or stream that flows into a larger one and is unrelated to the concept of sediment deposition at the point of a river's entry into a larger body of water.