Historical Fiction, Adventure, Quest
Technically, Treasure Island is an example of historical fiction, because it's set in the 1700s, a century before it was published. Still, the "historical" part of this novel's genre is much less important than the fact that it's an adventure. Robert Louis Stevenson conceived of Treasure Island as a plot-rich book for boys, in the tradition of other adventure novelists like James Fenimore Cooper (who wrote The Last of the Mohicans). What's more, Jim's adventure is a quest for treasure, so putting Treasure Island in the quest genre is pretty much a no-brainer.