Parallelism in literature is the use of a similar gramatical structure in order to transmit an idea in an easier way and to emphasize in certain points. For example, the parallelism is this excerpt is the use of "Adenauer, Erhard, Reuter, and other leaders..." and "The German leaders—the German leaders", because these two parts refer to the same people.
Reagan uses this device to introduce his reasoning and create emphasis, option C, in how the German leaders that adopted Capitalism were able to succeed economically and bring stability to West Germany.