Concept:When soil is shaken with water in a jar and left to stand, the particles settle in layers: the largest and heaviest settle first and the lightest stay on top.
Explanation:From the bottom upwards the layers are: gravel, sand, silt, then clay (as a cloudy suspension in the water).
Organic matter (humus and plant remains) is lighter than water and floats on top, above the clay suspension.
Gravel and silt settle below the clay, and loam is a soil type (a mixture), not a layer.
Answer:Option B (Organic matter)