Why are alluvial plains of Himalayan rivers highly fertile?

Correct answer: Because fine alluvial soil brought by rivers is deposited there

The direct answer is A: fine alluvial soil brought and deposited by rivers. Rivers erode rocks and soil in their upper and middle courses. During floods or slower flow on plains, the river loses energy and lays down layers of fine silt, sand and clay. These alluvial materials contain useful mineral particles and are often renewed by later floods, so they support productive agriculture when water and management are suitable. Option A is correct because it directly explains both the origin of the soil and its fertility. Option B is wrong because hard lava is volcanic rock, not the normal material forming Himalayan alluvial plains. Option C is wrong because rivers do flow through and build these plains. Option D is wrong because sea salt is not always deposited there and excess salt would often harm crops. Memory cue: alluvial plain means river-made deposit and usually fertile farmland.