Use the drop-down menus to identify the types of convergent boundaries labeled a, b, and c.

Oceanic-continental convergence occurs when an oceanic plate and a continental plate converge. They are pushed together and the oceanic plate is forced to go under the continental plate. So based on the diagram, this convergence occurred in letter B. 

Oceanic-oceanic convergence occurs when two oceanic plates collide into one another. The oceanic plate that is older, denser and/or colder will be the plate that will go under the other. It will be forced down into the mantle. So in our diagram, this occurs in letter A.

Continental-continental convergence occurs when two continental plates converge. In this case, the plates push up against each other and create mountain ranges. Unlike the other two, they do not sink down, the movement is upwards. This occurs in the area C of your diagram.