A stream terrace is best described as:

A stream terrace is best described as a raised flat area alongside a river. This geological feature forms primarily due to the processes of erosion and sediment deposition associated with river dynamics. When a river cuts into its floodplain, usually during periods of downcutting, it can leave behind abandoned, flat surfaces at varying heights above the current stream level. These surfaces represent former floodplains or areas where the river once flowed, now elevated due to tectonic activity or changes in base level.

Stream terraces can reveal a history of river activity, showing how a river has changed over time. They are significant for understanding landscape evolution, sediment transportation, and can provide valuable insights into soil development and ecological practices in the surrounding areas. The correct answer captures the essence of what a stream terrace is—its position in relation to a river and its flat, elevated nature.