Question

A memoir describes past events, but it is not considered a historical document. Why?

Answer 1

A memoir describes the biography that is personally written in which all life events are captured in words with his memory so it is not considered historical document because
  a memoir relies largely on the writer's memories
because it rely on writer memory and he can also forget something
so correct option is C 
hope it helps

Answer 2

Answer:

Option C.

Explanation:

Although a memoir can take place in a historical perspective, it is not a comprehensive recounting of a historical accident, but an anecdote of a person's personal and individual encounters, memories and views on a selective significance in his/her time, and how these visions altered the life of the author. The memories could have appeared on a historical context but they do not record in particular the context of the events around the artist, but his/her beliefs and ideas and how he/she encountered said events.