Which geological feature is created from the accumulation of sediment deposited by a river?

The correct choice describes alluvium, which is indeed formed from the accumulation of sediment that a river transports and deposits. As rivers flow, they erode material from their banks and beds, carrying it downstream. When the velocity of the river slows down, such as when it enters a wider area or during periods of decreased flow, the river loses its capacity to carry sediments, causing them to settle. This deposited material is called alluvium, characterized by its fertile qualities, making it particularly important for agriculture and ecosystems along riverbanks.

Other choices reflect different features related to river processes. River cliffs are steep edges formed by erosion, while slip-off slopes are gently sloping areas that occur on the inside bends of meanders where sediment is deposited, but they do not refer to the accumulation in a broad sense like alluvium does. Flood plains are flat areas adjacent to rivers that are periodically inundated, resulting from sediment deposition during floods, but again, alluvium specifically refers to the sediment itself rather than a landform created from it.