Answer:
A newspaper article written by a reporter who was in the audience at Ford's Theatre would be considered a secondary source about the assassination of Abraham Lincoln.
A secondary source is a source of information that is one step removed from the original event or period being studied. It is created after the fact, often based on primary sources, and provides interpretation or analysis of the original event.
In this case, the newspaper article was written by a reporter who was not present at the assassination but was reporting on it after the fact, based on information gathered from primary sources such as witness accounts. Therefore, it is considered a secondary source.
The other options listed in the question - a diary entry by a witness to the assassination, an interview with a member of the audience at Ford's Theatre, and an interview with Booth's understudy who was not at the theater that night - would be considered primary sources, as they are firsthand accounts of the event or period being studied.