Final answer:
The weakness that caused Austria-Hungary to struggle in World War I was the diverse nationalities within the empire seeking independence, which led to internal revolts and discontent.
Step-by-step explanation:
The weakness that caused Austria-Hungary to struggle in World War I was B. Diverse nationalities within the empire seeking independence. Austria-Hungary was a divided empire with various ethnic groups and nationalities, such as Serbs, who wanted to break free from imperial rule and establish their own independent nations. This led to internal revolts and rebellions, putting a strain on the empire's stability and resources. Additionally, the assassination of the archduke (C) and the lack of strong allies (D) further weakened Austria-Hungary's position in the war.