The correct answer is - D) the development of steel tools.
This early societies managed to have complex economies because they invented and started using the steel tools. This types of tools enabled them to create more, to significantly widen the spectrum of what they can produce and the quality of it. That gave them an advantage in trade too, so their economies were prospering and over time became complex and strong economies.
The highest civic virtue in Rome was to put the well-being of the state and the republic above your own. The founding fathers believed that the US would prosper if people behaved in the same manner and fought together for a higher cause.
