You could be describing either headwaters of a stream, or a waterfall.
A stream leaving a mountain range is called an “outlet” or “mouth.”
Alluvial fan
valley or foothill plain.
Option c is correct
Alluvial fan
alluvial fan
Alluvial fan
Valley
an alluvial fan.
As a stream leaves a mountain range, it may encounter flatter terrain and lower elevations. This can lead to the formation of valleys, plains, or even wetlands, depending on the local geography. The stream may also start to meander and slow down as it transitions from its mountainous origins.
This is known as an Alluvial Fan. Please see related links.
A row of mountains is called a mountain range.