Which fault is primarily caused by lateral motion between two rock masses?

The correct choice describes a strike-slip fault, which is characterized by lateral motion along the fault line. In a strike-slip fault, two blocks of the earth's crust slide past one another horizontally. This type of faulting occurs due to shear stress, which causes the rocks on either side of the fault to move sideways relative to each other, rather than up or down.

This contrasting motion is a distinguishing feature that sets strike-slip faults apart from normal and reverse faults, which primarily involve vertical movement. In normal faults, the hanging wall moves downward relative to the footwall due to extensional forces, while in reverse faults, the hanging wall moves upward relative to the footwall due to compressional forces. Thrust faults are a type of reverse fault, but with a shallower angle, also producing mainly vertical movement.

Given this context, the lateral motion in a strike-slip fault is what makes it the correct answer.