Final answer:
The O horizon is characterized by undecomposed plant material at the top, which includes humus and decomposed vegetation. The A horizon is considered the topsoil, while the B horizon is known as the subsoil.
Step-by-step explanation:
The soil horizon characterized by undecomposed plant material at the top is the O horizon. This layer has freshly decomposing organic matter, known as humus, at its surface, with decomposed vegetation at its base. Humus enriches the soil with nutrients and enhances soil moisture retention. The A horizon is the topsoil and is typically darkly colored because of the presence of organic matter. The B horizon, also known as the subsoil, is characterized by an accumulation of mostly fine material that has moved downward.