Answer:
C. provide firsthand information about a historical event.
Step-by-step explanation:
You want to know why a primary source is valuable in the study of history.
Primary source
A primary source is a timely first-hand account, written or recorded by someone who witnessed or participated in the events being recorded. For the reason that it is unaffected by those who would interpret the events through the lens of their own agenda or bias, such a source is valuable to historians.
Sometimes such a source will refute generally accepted conclusions about the past. It is usually the work of a single person. Other sources may corroborate the view, or they may have a different viewpoint.
Primary sources are valuable because they provide firsthand information.
__
Additional comment
Technology being what it is, primary sources these days may include audio, photographic, or video recordings of events, as well as written records (diaries, letters, articles).