B. By comparing the source to other established sources from that time period
Explanation:
A source is considered as authentic when the source is genuine which means it does not copy other sources and also when this is accurate and has been not modify with biased purposes. In primary sources, which are first-hand account sources, this quality can be proven by historians and experts by analyzing whether the elements of the source fit with the time in which they were written and the possible author, which can be done by comparing the source being studied to sources that were created during the same period of time and that have been proven as authentic, as authentic sources written during the same time share common features. Therefore, one way in which historians can establish the authenticity of a primary source is by comparing the source to other established sources from that time period.
