I think this question is not worded correctly, as three of these four things are caused by river erosion. River deltas are obviously caused by river erosion. Oxbow lakes occur when a meander of a river (caused by river erosion) is isolated when the river changes course. Alluvial fans are also formations similar to deltas caused by river erosion. The only one which is NOT formed by river erosion is (c) SOIL - soil is formed from inorganic material mixed with the decomposed remains of organic matter.
