In the soil profile, the remains of plants and animals are present in a layer known as the humus layer. The topsoil or the A horizon refers to a zone of leaching and the habitat of various species. The subsoil or the B horizon refers to a zone where the organic matter and soluble salts are leached from the A horizon is accumulated.
The weathered part of the rock, which produces the parent substance over which a soil establishes makes the C horizon, and the unweathered part of the parent substance forms the D horizon. Thus, from the given diagram it is clear that the layer Y comprises unweathered part of the parent material.
