Final answer:
The correct answer is a. topsoil, which is the A horizon and the uppermost soil layer rich in organic material where most microbial activity occurs, essential for plant growth.
Step-by-step explanation:
The highest layer of dirt from which plants take nutrients is known as topsoil. This is the uppermost layer of the soil, also referred to as the A horizon. This layer is rich in organic material and is where most of the microbial activity in soil takes place, making it the critical layer for plant growth. Conversely, subsoil refers to the B horizon, which is a layer beneath the topsoil that accumulates material leached from the upper layers. Bedrock, lying beneath the C horizon or soil base, is the solid rock that makes up Earth's crust. Humus refers to decomposed organic matter that enriches the soil with nutrients and helps retain moisture. Considering the options provided (a. topsoil, b. subsoil, c. bedrock, d. humus), the correct answer to the given question is a. topsoil.