The correct answer is E. Useful primary source document
Explanation:
In history, a source refers to any source or material that provides information about an event of the past. This source can be classified as primary sources if they were written during the historical event studied and therefore are a first-hand account of the event or as secondary sources if the source uses primary sources to create a new source. Also, sources are considered as useful or effective if they provide information about a historical event. Therefore, a letter written during the time a historical event occurred would be considered as a "useful primary source document" because this document is a first-hand account of the historical event being studied (primary source) and also provides information about it, which make it a useful document.
