Final answer:
The Horizon O or Organic Horizon contains the highest concentration of organic matter.
Step-by-step explanation:
The horizon that contains the highest concentration of organic matter is the Horizon O or Organic Horizon. This horizon is usually found at the surface of the soil profile and consists of freshly decomposing organic matter, such as leaves, plants, and animal remains. The organic matter in the Horizon O enriches the soil with nutrients and enhances soil moisture retention, making it a crucial layer for plant growth.