Which literary device is the main message conveyed to the reader through the plot of a text? plot climax theme symbolism

The correct answer is theme.  

Obviously it isn't plot, as plot is used in the definition/clue for the word.
If you have studied figurative language, you should know that it can't be symbolism.
Also, while the climax should be invigorating, it shouldn't be the main message of the entire book, and it isn't really conveyed through the plot.

By process of elimination, you could figure out that this was theme.