Which horizon is formed by the accumulation of clayey materials stained by minerals?

The B Horizon is typically characterized by the accumulation of materials that have leached down from the layers above, particularly from the A horizon, which is rich in organic matter. One of the key features of the B Horizon is the accumulation of clayey materials, which can give it a denser and often more compacted structure compared to the layers above.

In many soils, this horizon can also present staining from minerals, particularly iron oxides or other compounds that contribute to its color and characteristics. This mineral staining occurs due to the processes of weathering and soil formation, where water movement through the soil profile facilitates the deposition of soluble minerals from upper horizons. Thus, the B Horizon serves as a crucial layer where materials leached from above, along with clay and minerals, accumulate.

The other options represent different layers within the soil. The O Horizon consists mainly of organic matter like decomposed leaves and plants, the A Horizon, often referred to as topsoil, is rich in organic matter and is where most biological activity occurs, while the C Horizon is made up of partially disintegrated rock and mineral materials, not yet transformed into soil. Thus, the characteristics of the B Horizon align perfectly with the description given in the question.