Answer:
D. Analyzing historical sources
Explanation:
In order to understand well what historical sources are, it is necessary to know that the word "source" here is understood in the sense of "document", that is, something that records the testimony of some event that occurred in the past . In this sense, historical sources or documents are all that human beings have produced in the past, such as the letter the historian found.
As the historian is translating the letter and finding historical details and evidence, we can conclude that the historian shows ability to analyze historical sources.