Which soil horizon contains a layer of partially weathered or unweathered rock?

Option (B)

Explanation: The R-layer represents the unweathered rocks that are located below the surface and is the lowest layer in the soil profile. Here, hard rocks are found which are difficult to break, and are breakable with heavy machineries only. This R-layer makes the foundation of the soil profile that supports the entire overlying series of layers.

Thus, the correct answer is option (B)