Final answer:
When it rains, the A horizon or the topsoil typically becomes the thickest due to its composition and dynamic interaction with rainwater and organic material, making it richer and stimulating its growth.
Step-by-step explanation:
The thickness of soil horizons can vary with various environmental conditions, including rainfall. When it rains, the A horizon, which consists of a mixture of organic material with inorganic products of weathering, is influenced the most. Typically, known as the topsoil, the A horizon is where most plant root activity occurs and is rich in nutrients. It can become thicker due to the addition of decomposed organic matter and by the percolation of water that carries materials from the surface. This layer can be significantly deepened in areas with heavy rainfalls.
In contrast, the O horizon, which is primarily composed of freshly decomposing organic matter or humus, may also increase in thickness after substantial rainfall as it supports more plant growth and subsequent accumulation of organic debris. However, the A horizon is generally more impacted by rain infiltration and has a greater potential to become thicker, as it combines organic matter from the O horizon with inorganic materials moved by water from the surface.
The answer to the student’s question depends on the specific environmental conditions and soil development processes. Yet, typically, the A horizon (topsoil) is the one that can become thickest when it rains due to its composition and place in the soil profile.