Which best identifies a defining characteristic of an autobiography ?
1( it often includes a futuristic setting.
2( its purpose is to recount historical events
3( it always uses the compare and contrast text structure
4( it is written by the author about his or her own life

The answer that best identifies a defining characteristic of an autobiography would be 4(it is written by the author about his or her life.
Auto means self and biography is a story about someone's life, so if you put the two together, it is a true story about the author's life, written by the author.