Concept:Loess is a fine, wind-borne deposit of silt and clay-sized particles.
Explanation:The question describes a layer formed by very fine wind-deposited material such as sand and silt.
Among the options, only loess matches this description exactly.
A seif is a long, sharp-crested sand dune shaped parallel to the prevailing wind.
A barchan is a crescent-shaped sand dune with its horns pointing downwind.
A bajada is a broad apron of gravel and sand found at the base of a mountain range.
When wind transports fine silt and sand over long distances, it deposits them as a smooth, uniform layer called loess.
This deposit is common in semi-arid regions and is well known in parts of North-West China.
Therefore, the correct answer is loess.
Answer:A. loess