Which is the best example of a secondary source document useful in the study of history?

The best example would be a textbook. That is because a secondary source is somebody's analysis of the primary source. For example, a thing happened like a battle and we know about it from letters and pictures of people. A text book would be made by a writer who would describe this and analyze it and it would therefore be a secondary source.

C) the conditions in the factory have never been questioned by the employee.