Based on the options provided and the context of mid-ocean ridges, the best description for the process shown in the diagram is:
A.) magma moves upward and crystallizes, forming a rock.
Mid-ocean ridges are formed as tectonic plates pull apart, allowing magma from the mantle to rise and create new oceanic crust as it cools and crystallizes. If the diagram illustrates magma activity at a mid-ocean ridge, this option aligns best with that process.