River deltas are
formed by erosion
created by seafloor spreading
caused by the movement of tectonic plates
areas of infertile, rocky soil

Answer;

formed by erosion

River deltas are formed by erosion.

Explanation;River delta refers to a landform that forms from deposition of sediment carried by a river as the flow leaves its mouth and enters slower-moving or stagnant water. In other words river deltas are wetlands that form as rivers empty their water and sediment into another body of water, such as an ocean, lake, or another river. It occurs when a River is entering a sea, an ocean, lake a reservoir or another river that is incapable of carrying the supplied sediment. Therefore, deltas are formed as a result of erosion by water which involves the carrying of sediments down the rivers to the mouth of river where it accumulates.