Answer:
a letter written by a person who is describing a current event
Explanation:
A primary source is a document from the time period of the event.
While most of these could be considered one, you have to do a process of elimination.
A scholarly article about an old event isn't a primary source, so it's not that.
An interview with a reporter who wrote about a current event is not a primary source because it is not the original document from the reporter.
A student's report about an event that occurred during her lifetime is a primary source, but it is not the best example.
