Concept:A waterfall forms where a sudden change in the gradient of a river bed forces water to drop steeply.
Explanation:Waterfalls commonly occur when a river flows over a hard, resistant rock layer that rests on a softer rock layer beneath it.
The softer rock erodes more quickly, undercutting the harder rock above.
This process creates a sharp break, or step, in the bed of the river.
Water then plunges vertically or flows steeply down this ledge, forming a waterfall.
Among the given options, the presence of a sharp break in the river bed is the direct condition that produces a waterfall.
Elbow of capture, backward erosion, and lateral erosion are related to river processes but do not directly create waterfalls.
Answer:A. sharp break in the bed of a river