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.
The theme of a text refers to the main idea that it tries to communicate to the reader. You cannot describe a theme in many words. Usually the theme can be identified in 1 to 3 words.
Fables are famous for being the easiest text for identifying a theme.
For example in The Lion and the Mouse, the theme is Loyalty and Trust.
That is the message that the author wants to communicate to the reader when the characters make favors to each other that saves their own lives.
