Which of the following horizons is typically more compact than Horizon A?

Horizon B is indeed typically more compact than Horizon A. In soil profiles, Horizon A, often referred to as the topsoil, is characterized by high organic matter content, soil structure, and biological activity, which contribute to its looser, more porous nature that allows for good water drainage and root penetration.

In contrast, Horizon B, also known as subsoil, experiences the accumulation of minerals leached down from Horizon A. This accumulation, along with a higher density, typically results in a more compact structure. While it can contain clay and various minerals, its density can limit root growth and water movement compared to the upper horizons.

The other horizons mentioned—Horizon O is mainly composed of organic matter and decomposing plant material, making it less compact; Horizon C consists of partially disintegrated rock material and is generally unconsolidated; and Horizon D is not a standard classification in soil horizons. This context illustrates why Horizon B is the correct answer, as it directly reflects the compaction that occurs below the more loose and organic-rich Horizon A.