An autobiography differs from a biography because it is written by

Autobiography is a story about rhe author and biography is a story about another persons life.

An autobiography is written by the person about themselves. For example if you wrote a story about yourself it would be an autobiography. 

A book written by someone, about someone else is a biography. For example, If I wrote a book about your life.

"An autobiography differs from a biography because it is written by" - the person the book is about. / the person themself

What’s the choices

Sometimes when you read things in your head, you skip overall small details/words. when you read aloud, you hear those words and it’s easier to comprehend what you just read. saying things aloud also you remember them.