Answer:
a. promote a single German culture
Explanation:
Bismarck's policy of Kulturekampf was an attempt to: "promote a single German culture"
This is based on the idea that Kulturekampf means culture struggle. And to ensure its realization, the policy favored the strengthening of the German army, it also tried to unify all of Germany and at the same time made and broke alliances as necessary.
Hence, the correct answer is "promote a single German culture"