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 professorial analysis of key battlefield strategies of the event

d. a journal excerpt from a person involved in the event in question

A primary source document refers to a document that was written during or shortly after an event being studied; additionally this sources are written by someone who directly experienced or witness the event, because of this primary sources are especially useful in history because they provide information that is authentic and is first-hand information, which made them different to secondary sources that are written based on primary sources. Considering this, the document that is a primary source and is useful in the study of history is "A journal excerpt from a person involved in the event in question" because this source was written during or shortly after the event studied and by someone that was involved which means is a first-hand account of the event or a primary information. Also, other documents are secondary as they do not directly report the event or were not written by someone who experienced it.