Which layer of soil is closest to the surface?

a. O (organic matter)

b. A (topsoil)

c. B (subsoil)

d. C (parent rock)

The layer of soil closest to the surface is letter a: O (organic matter). This is the top, organic layer of soil, mostly composed of unconsolidated mineral, dead leaves and humus - a decomposed organic matter - which functions as a natural avenue for growth of land plants.