The other answer can't be right... I'm looking at a compare and contrast chart in my textbook that says both the Roman empire had a senate.
The answers are "centralized absolute power in one ruler" and "instituted a census for taxation purposes".
An excerpt from my textbook on the second answer:
"Augustus also undertook economic reforms. To make the tax system more fair, he ordered a census, or population count, of the empire so there would be records of all who should be taxed."
Augustus was the first emperor of the roman empire! The change from the republic to the empire started with him, and the citizens didn't even know it was happening! In the empire (and not the republic), there was a census (population count) to be sure that everyone who needed to be taxed would be.
As for the other choices...
The empire didn't reduce trade or pull back from expansion at all! Also, there was certainly a senate. And there was no election of emperors. They inherited their power.
I attached the chart from my textbook comparing the republic and the empire for anyone reading this who needs help! :)