Which of the following tactics allowed the classical persian empire to unite its vast territory under a single government?

A tactic the Persian empire used to unite its vast territory under a single government is: 

setting up smaller proxy government that is runned by the people of the land. 

As long as they payed their taxes and did not break any laws that are set (and not rebel), they were able to do mostly whatever they did before. They were able to keep their own religious laws, choose to either return to their homeland or settle in a different area, etc.

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