Which action of a river mainly helps in forming floodplains?

Correct answer: Deposition

The direct answer is A, Deposition. A floodplain is a relatively flat area beside a river that may be covered by water during floods. When a river floods, its water spreads outside the main channel and becomes slower. Because slower water has less carrying power, it drops silt, sand and other fine material on the nearby land. Repeated deposits gradually build the floodplain and may make its soil fertile. Option A is correct because deposition means the laying down or settling of transported material. Option B, a volcanic eruption, can produce lava and ash but is not the normal river process that forms a floodplain. Option C, a seismic wave, is a vibration caused by an earthquake and does not spread river sediment to form this landform. Option D, an avalanche, is a rapid fall of snow, ice or rock down a slope and is unrelated to floodplain formation. The fact chain is: flood, slower water, sediment settles, layers accumulate, floodplain forms. Remember: a floodplain is mainly a depositional feature beside a river.