One of the primary differences between the study of society done by the people of ancient civilizations and the study of society conducted by sociologists in the nineteenth century was that

The correct answer to this open question is the following.

One of the primary differences between the study of society done by the people of ancient civilizations and the study of society conducted by sociologists in the nineteenth century was that they were able to test their theories using systematic research never done before.

When we are talking about systematic research we imply that the research is done in a meticulous process dividing each step in the process of investigation. The research has different steps that the researcher is covering one by one to reach a conclusion at the end of the study. The structured analysis in systematic research allows scientists to be more exact in their conclusions.

d: social contract theory