Which soil horizon is least likely to interact directly with living organisms?

The C horizon is composed primarily of weathered bedrock and parent material from which soil develops. It contains little organic material and is typically less biologically active than the other horizons. Because of its location beneath the more biologically rich layers of soil, the C horizon has limited direct interaction with living organisms.

In contrast, the O horizon is rich in organic matter, including decomposed leaves and other plant material, making it highly interactive with organisms like fungi, bacteria, and insects. The A horizon, or topsoil, is the most fertile and biologically active layer, filled with organic material and nutrients, thereby attracting a variety of living organisms. The E horizon is characterized by leaching and loss of minerals, but it still has some level of interaction with organisms that may be involved in nutrient cycling. Thus, among these options, the C horizon stands out as the layer least likely to engage with living organisms.