The correct answer is C) an editorial from the Charleston Mercury in 1860.
The primary source document that could help a historian understand why the Civil War started is an editorial from the Charleston Mercury in 1860.
Let's remember that a primary source is the kind of source written by an individual that was part of the event in history, that witnessed how the situations unfolded. In this case, an editorial from the newspaper "Charleston Mercury" in 1860, depicted the situation that the state and the nation were living in a year previous to the beginning of the Civil War. Another primary source could have been a photograph taken by the journalists of the time or an interview with some government officials in that time too.
