Which is the best example of a primary source document useful in the study of history?
A diary or personal account that was written during the time of the event, etc.
Without a set of options in front of me, my best guess would be something along the lines of my above answer. Primary sources are typically diaries, legal codes, and other firsthand accounts/sources of information. They typically provide the most accurate account of the event that occurred because they have not been interpreted further by any other people.