Sentinel because it means a person or a thing watches or stands as if watching.
Can you show the passage??
The answer is B. it's just a way to strengthen your argument by debunking the other side respectively.
The part that best demonstrates the author's purpose is to entertain children with a humorous story is: "Till at last there was only one small fish left in all the sea, and he was a small 'Stute Fish, and he swam a little behind the Whale's right ear, so as to be out of harm's way.