Concept:Different crops require specific soil types based on drainage, aeration, and nutrient content.
Explanation:Desert soils are sandy, low in humus and moisture, and not suitable for paddy (a water-loving crop).
Sand and loamy soils are well-drained with good aeration, making them ideal for onion cultivation.
Black regur soils have high water-holding capacity and are best suited for cotton, not greengram.
Red soils, containing iron oxide, are typically used for wheat, pulses, tobacco, and millets, not optimally for cotton.
Among the given options, only option C correctly matches a crop to its suitable soil.
Answer:C. Sand and loamy soils are best suited for onion crop.