The difference between autobiography and memoir is

a. autobiography covers the writer's entire life; memoir covers a particular time in the writer's life or events around a theme
b. autobiography is written by a person about his or her life; memoir is written about the person by someone else
c. autobiography is an oral biography; memoir is written
d. virtually nonexistent

An autobiography is a biography written by the person who is writing about themselves. A memoir is written by someone else about a different person, someone they knew or were intimate with; based on personal observation.
I would say the answer to be B, out of all of them. A is a bit vague, because both can cover their entire lives, obviously not when they're dead though. C isn't true because both can be oral and written. And D doesn't make sense. 

A, this way you know that there’s a less of a chance of it having biased information or edited by other people-making false information