Final answer:
The O horizon or A horizon contains the most organic matter in the soil.
Step-by-step explanation:
The soil horizon that contains the most organic matter is the O horizon or the A horizon. The O horizon, also known as the surface horizon, consists of freshly decomposing organic matter called humus, which enriches the soil with nutrients and enhances moisture retention. The A horizon, or topsoil, contains a mixture of organic material and inorganic products of weathering, making it rich in organic matter.