The topsoil (A horizon) of the soil profile is most affected by weathering and erosion due to its high organic content and position at the surface where these processes are most active.
The layer of the soil profile that would be most affected by weathering and erosion is the A horizon, also known as the topsoil. This layer consists of a mixture of organic material with inorganic products of weathering, making it particularly susceptible to the forces of weathering and erosion. It is typically darkly colored because of the presence of organic matter, and is crucial for plant production as it is rich in nutrients.
In contrast, the B horizon is considered the subsoil, and the C horizon is known as the soil base, which is less affected by surface processes due to their depths. The O horizon consists of freshly decomposing organic matter and is less likely to be transported by erosion compared to A horizon. Therefore, the main answer to the question is option B, the A horizon or topsoil.