'Imphal Basin' surrounded by Manipur hills is a fine example of

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Correct — A, lacustrine plain. A lacustrine plain is the flat floor left behind when a lake is filled in with sediment or drained away, and the Imphal basin is the standard Indian textbook example. It is an oval, remarkably level intermontane valley in central Manipur, lying at roughly 790 m above sea level and ringed on every side by the north-south Manipur hill ranges. Enclosed basins of this kind trap the drainage of the surrounding slopes, and this one held a large lake through much of the recent geological past. Silt washed in from the encircling hills gradually filled it, leaving the smooth, waterlogged floor that carries almost all of Manipur's settlement and wet-rice cultivation on about a tenth of the state's area. Loktak Lake, in the south of the basin, is the shrunken survivor of that former water body — the largest freshwater lake in North-East India, a Ramsar site since 1990, celebrated for its phumdis, the floating mats of vegetation, soil and decaying organic matter on which fishermen build huts. The presence of a residual lake inside a flat, hill-girt basin is precisely the signature of a lacustrine plain.

Why the other options are wrong — (b) Loess is fine, wind-blown silt, typically swept off glacial outwash or desert margins and dumped in thick unstratified blankets — the Loess Plateau of northern China along the Huang He is the classic case. It requires a dry, wind-dominated regime with a large dust source. Manipur is a wet, densely forested, monsoonal hill country with no such source, and no significant loess deposits exist there.  (c) Glacial plains are till or outwash surfaces left by ice sheets or valley glaciers, as in the North European Plain. The Imphal basin sits at only about 790 m in the sub-tropical North-East, far below any Pleistocene snowline in this latitude, and shows none of the moraines, erratics or outwash fans that a glacial origin would leave.  (d) The intended trap, and the one that is partly true. The floor of the basin does carry alluvium brought down by the Imphal, Iril, Thoubal and Nambul rivers, so a candidate who sees flat, fertile, river-watered land reaches for 'alluvial'. But an alluvial plain is built by rivers depositing across an open plain, while this is a closed intermontane depression whose flatness comes from the infilling of a standing lake — with Loktak still there as the remnant. Indian physiography texts therefore classify it as lacustrine, and the question's phrase 'surrounded by Manipur hills' is the deliberate clue that the basin is enclosed.

Concept: Plains are classified by the agent that made them: rivers give alluvial plains, wind gives loess plains, ice gives glacial (till and outwash) plains, the sea gives coastal or marine plains, and standing water gives lacustrine plains. A lacustrine plain forms where a basin holds a lake long enough for inflowing streams to lay down horizontal beds of silt and clay across its floor; when the lake finally silts up, is drained by headward erosion, or shrinks with a change in climate, that floor is exposed as an unusually flat, fine-grained, often poorly drained plain. Enclosed intermontane basins in tectonically active hill country are ideal settings, which is why the Imphal valley of Manipur and the Vale of Kashmir are India's two textbook lacustrine plains.

State PCS 2019 Q123 — Geography PYQ with Answer

State PCS 2019 Q123 — Geography PYQ with Answer