Under which condition can alluvial fans form near the upper Himalayan river course?

Correct answer: When the river leaves mountains and slope suddenly decreases

An alluvial fan forms where a river carrying sediment leaves a steep mountain valley and reaches a flatter area. The sudden reduction in slope causes the river to lose speed and transporting power. It then spreads outward and deposits gravel, sand, and finer material in a fan-shaped body, commonly near the foothills. The fan may have several shallow branching channels because the river changes course across its deposits. The amount and shape depend on sediment supply, water flow, and local relief.

The correct choice is A: when the river leaves the mountains and the slope suddenly decreases. This condition encourages deposition because the river can no longer carry all its sediment efficiently. The other choices do not provide the necessary change in slope and sediment movement. A river rising from the sea is not the normal setting, a sediment-free river cannot build an alluvial fan, and a river does not become a volcano. The key link is steep slope to gentle slope plus deposition.