The diagram is representative of an area such as the Mid-Atlantic Ridge. The arrows indicate that the plates are coming apart or diverging. This divergence opens a window to the mantle allowing magma to come onto the seafloor forming a ridge which spreads as the plates continue to diverge. Therefore, the logical process being illustrated is B.

Clearly, the answer cannot be D because the plates are not converging nor is there a vertical displacement of the crust that is associated with reverse faulting (A) nor uplift due to isostatic adjustment (C).