A is divergent plate boundary, B is a transform plate boundary, and C is Convergent plate boundary. A divergent boundary happens when a linear feature between two tectonic plates are starting to move away from each other.
This means that the rift will get wider until they are truly separated from each other. Transform plate boundary occurs when there is a sort of fracture that appears mid-ocean.
This would seem to separate the ocean into two parts. Convergent plate boundary refers to the volcanoes that are located above the melting oceanic plate. There are some known convergent plate boundaries in the world like the Cascade Mountain Range.