Answer: d. C (parent rock)
Soil horizons are the layers lying parallel to the surface or subsurface layers of soil. These horizons are formed in the process of soil horizon. These sequence of soil horizons is collectively called as soil profile.
The layer or soil horizon, which is least likely to be penetrated by plant roots is C horizon. The C horizon is a layer of soil, which consists of huge aggregates of parent rocks, which are unweathered or partially weathered, lying deep in the bottom of the soil profile. It does not contain organic matter. Also, the texture of the soil is not fine, instead hard rocks are present. Therefore, this layer of soil cannot be penetrated by plant roots.
