The correct answer is C) Understanding the author's use of structure and rhetoric.
The statement that is a way for a reader to better understand why an author shares certain ideas within a primary source is "Understanding the author's use of structure and rhetoric."
This way the reader can get a better idea of why the author is using a primary source. It could be an element to identify the date of the event. It could be a testimony that confirms the series of incidents that led to something else. It could be an interview with a character that witnessed the event or lived in the time of the narrative.