Answer: D) A theme is the purpose of a text.
Step-by-step explanation: In literature, the theme is the underlying message of a story, it is what critical belief about life is the author trying to convey in the writing of a novel, play, short story or poem. Usually this belief, or idea, is universal and transcends cultural barriers. From the given options, the one that represents the definition of a theme in literature, is the corresponding to option D, the theme is the purpose of a text (the message).