Answer:
B. Austro-Prussian War.
Step-by-step explanation:
Since the idea of finding a heading on the first line of the list regarding the Unification of Germany, then the Austro-Prussian war fills the gap.
The Austro-Prussian war was a conflict in 1866 deliberatly provoked by then Chancellor Otto Von Bismarck with the means of expelling Vienna from the German Confederation. His intent was to unite Germany exclusively under Prussian Dominance.