This is not only the title of William Styron's novel but refers to a central plot point and revelation, one that affects the title character's entire life. The novel is told from the point of view of a young man named Stingo who meets Sophie because she lives in his apartment building shortly after World War 2 ends. They become friends and he slowly learns more about her mysterious past. Sophie's live-in partner and lover is an abusive narcissist named Nathan, but Sophie loves him deeply.

Sophie's "choice" could possibly refer to her choice to be with Nathan even though he treats her horribly and has a dangerously violent temper. This speaks to her troubled past and the emotional vulnerability she feels as a result of going through traumatic events during the war. Another "choice" she makes is to have sex with Stingo, as a way of finding comfort after she and Nathan have a fight and Nathan leaves.

But the central "choice" being referred to is the choice she is forced to make by an armed Nazi guard, who says she must chose which of her two small children survives: her son or her daughter. Of course this is an impossible choice fora mother to make, but if she does not choose, the guard will order both of them killed. It is a horrific act of torture because of course Sophie will be haunted by this traumatizing event her entire life. She chooses for her daughter to be taken, for her son to survive, and it's impossible to say why she chose this way, aside from the assumption that if her daughter dies then she won't ever have to experience the agony of giving birth to a child who is taken away and killed. But the novel doesn't really clarify why Sophie chooses as she does.