Answer: D
The answer is D because the information following "message" is the message. A colon is appropriate (as used in D) when the information after the colon describes or represents the word or phrase that comes before the colon.
As the title suggests, a treasure, but more specifically alluded in the plot, it's the pirates'treasure . However, a treasure is always the symbol to depict the immorality of that age when Stevenson wrote this novel of adventures.
If I'm not mistaken your answer is C :)