Concept:The A-horizon is the topmost soil layer, rich in organic matter, where most biological activities occur.
Explanation:The soil profile consists of distinct layers, or horizons, labelled A, B, C, and D from top to bottom.
The A-horizon is the topsoil, containing humus, plant roots, and abundant microorganisms.
Earthworms, bacteria, fungi, and other organisms are most active in this layer because it has plenty of food and oxygen.
Deeper horizons, such as B and C, contain less organic matter and support fewer biological activities.
Therefore, the layer with the greatest biological activity is the A-horizon.
Answer:D. A-horizon