In the study of history as an academic discipline, a primary source (also called original source or evidence) is an artifact, a document, a recording, or other source of information that was created at the time under study. It serves as an original source of information about the topic.
The correct answer is C. A Charleston Mercury editorial published in 1860
Explanation:
To begin with, a primary source is a source created during the event being studied by someone who witnessed or experienced it. This implies if a historian is trying to understand the causes of the Civil War through a primary source he needs to focus on sources that were created before 1861 when this war started.
Moreover, as the focus is the causes of the Civil War the best type of source is one that analyzes the situation of the American society around 1861 and depicts the factors that contribute to the war. Considering this, the best document is "A Charleston Mercury editorial published in 1860" because this is a primary source and through this, the historian can understand the social situation and factors that lead to the war, which is the focus of the historian.
