Fantasy; Adventure
If you find yourself stepping through a portal into an "otherworld" that has talking animals, species you've never seen before, and a similar yet slightly foreign social structure, you know you've entered a work of fantasy. It's also convenient if you have magical creatures (good or evil—preferably both), a dangerous quest to bring justice to the land, and redemption in the mix, all of which is true in The Silver Chair.
As for the adventure genre, well, doesn't everything we just said sound pretty exciting and risky? We certainly meant it to. And this, Shmoopers, means The Silver Chair is also an adventure.