Which is the best example of a primary source document useful in the study of history?
a) a documentary with expert analysis of the event in question.
b) a peer-reviewed article written about the event by a respected historian.
c) a professiorial analysis of key battlefield strategies of the event.
d) a journal excerpt from a person involved in the event in question.

The fourth alternative is correct (D).

History is usually narrated in a secondary way, through specialized observers of a particular context - historians.

However, although with less incidence, there are stories that are narrated by primary sources. The fact that it is a primary source means that the information comes from someone who has experienced that historical moment, that is, it is an in loco narration. In a way, it is a privileged view that is usually valued by historians.

An example of this may be a diary of someone who is involved in a war.

i believe the answer is c