C secondary source documents that are useful in the study of history
The correct answer is A. Primary source documents that are useful in the study of history.
Explanation
Letters and magazines can be considered primary sources by historians because there are documents in which they can find reliable information wrote by person who lived in a specific time or event that occurred in history, for example, the history of the countries of America has been constructed from Navigation charts or diaries written by the discoverers or conquerors. Also, it is a way to know how the great civilizations were and the life in them thanks to the letters written by the scribes, who were at the service of the emperors to issue messages to another emperor or the population of their kingdoms. So, the correct answer is A. Primary source documents that are useful in the study of history.
