What features are defined as accumulations of silt deposited at river mouths, eventually creating fertile ground?

The correct answer is indeed deltas, which are landforms created by the accumulation of sediment, primarily silt, deposited at the mouth of a river where it meets a body of water, such as an ocean or lake. As the river flows, it carries sediments downstream. When the river empties into a larger body of water, the velocity decreases, leading to the settling of these sediments and the formation of a delta. This process creates rich, fertile soil ideal for agriculture and supports diverse ecosystems.

In contrast, swamps, basins, and marshes are different types of wetlands characterized by specific features and conditions. Swamps are typically forested wetlands, whereas marshes are non-forested wetlands dominated by herbaceous plants. Basins generally refer to depressions in the landscape that can collect water but are not specifically tied to sediment deposition like deltas.