According to Bismarck what was Prussia's role

The correct answer is: to unify Germany.

Explanation:

Bismarck's main goal was to unify Germany under Prussian control. He considered Prussia should rule over united Germany, and not to only one of its provinces. So, we could say the unification was important to Bismarck because he wanted to achieve the expansion of Prussia more than the unification of Germany. After the unification, he became the first Chancellor of the North German Confederation.

He also believed that Germany's central position in Europe could be the main cause of its destruction in the event of any war, so Bismarck thought that Prussia was better positioned and more suitable to control united Germany.