Without seeing the specific diagram you're referring to, I can provide guidance based on common knowledge about the Incan civilization. The Incan Empire, known for its extensive road systems and administrative organization, was characterized by unification under a centralized authority, which helped in managing its vast territories.
Given the options you've provided, the most appropriate statement that identifies a major effect of the causes shown in a diagram about Incan civilization is likely:
unification of the empire
This choice reflects the overall impact of the Incan strategies and governance that brought diverse cultures and regions together under a single powerful rule.