Which of the following horizons is considered a region of subsoil?

A. O horizon

B. D horizon

C. A horizon

D. B horizon

Let's see. It's definitely not the O horizon, because that layer is at the very top. A is already eliminated. Same thing with the A horizon, that is actually the top soil. C is out. It has to be either B or D. A D layer doesn't exist, so B is out. But the B horizon is considered a region of subsoil. The answer is D: B horizon.