The theme is the moral or the lesson of the story. After reading the story, which of the answers gives you a lesson that is universal (can apply to anyone) and fits with the story. For example, one could say that the moral of Finding Nemo is "always listen to your parents as they know best". No specific details like names or places are given in the theme.
It helps develop ideas and mood at the appropriate speed.
It was in 1965 and in Tulsa Oklahoma but I don't know where it is in the book