The correct answer is A) external criticism.
The process that the historian followed when testing the accuracy of the source against other facts and outside data is called external criticism.
External criticism is a resource used by historians to validate the information of important documents in history. It has to review and confirm the authorship of the document. If it's a copy, how true and real is. Then, the historian has to double-check possible mistakes in the information. The purpose is to find that the document is valid.