B. a letter from him is your best answer
Primary sources are sources from the time and place, and generally is the best when it comes from the person himself.
A painting about him done today is not a primary source, because it was not created during the time of George Washington
A description about him from a historian is not a primary source, because:
1) The historian has never met Washington himself
2) The historian did not live in the same time period
A social studies textbook is not a primary source, because the text book draws it's information from other secondary sources and the primary source. Textbooks usually are made years after the events occured.
hope this helps