The correct answer is "Historical argument."
The historian's main point is an example of a historical argument.
A historical argument states the reasons to support a certain opinion about historical events that happened in the past. In this case, the historian who is writing the book about the Native Americans Indians' role in building the United States, has to make the correct arguments to demonstrate its point.
The historian should include more details and factual support.
The historian has to check if the information included is supported by primary or secondary sources. It should include data, names, numbers, statistics, and valid references. This way the historian can have more detailed information for its book.