A. A biography of Charlemagne written by his friend and attendant Einhard in the early 9th century.
A primary source is a firsthand or contemporary account of an event or historical figure, created at or around the time of the events or by someone directly involved. In this case, Einhard's biography of Charlemagne, written in the early 9th century, is a primary source because it is a direct account from that historical period and written by someone close to Charlemagne.