Answer: its central message
Explanation: The theme of a literary selection is the central message its trying to pass across.
For example, a book that talks about how greed ends up destroying a good relationship between two people, or how love heals all wounds is passing them off as the central message.
To identify the theme of a literary selection, there are important questions to ask such as, "what is the book about?", "what is the aim of the book?"