The correct answer is a) chronology.
The organizing principle of the list is a chronology.
When using chronology, you are listing the concepts in the order they were happening. You start with the first and ends with the last one. A chronology tells you what occurred in the first place, what followed, and what was the last incident, activity or deed. In this case, the Civil War was from 1861 to 1865, the Reconstruction was from 1865 to 1877, and finally, the Progressive era from 1890 to 1920s. This is a chronological list.
