A memoir...is no considered a historical document. Why?
C. because a memoir relies largely on the writer's memories.
Although a memoir can take place in a historical setting, it is not a complete retelling of a historical event, but a narrative of a person's inner and individual experiences, recollections and perspectives on a particular moment in his/her life, and how these memories changed the author's life.
The memories could have happened on a historical environment but they do not document in detail the context of the circumstances around the author, but his/her feelings and opinions and how he/she experienced said events.