The correct answer is A) by comparing the source to other established sources from that time period.
One way that historians can establish the authenticity of a primary source is by comparing the source to other established sources from that time period.
Historians know the importance to have a good and verifiable primary source in their research. That is why they have to double or triple check their sources. Historians could have a document from the time of the study or a photograph. Let's say, the Civil War. To validate it, the historian can compare this piece of information with others of that time to confirm the exact date, location or testimony before including it in its research.