The answer is A. if the dark thing is coming after the children.
It is a compound noun. Why? Because 'Back''Pack' is one word, but if you see how i structured it, it can be two. Back, as in your body part, and Pack as in a bag or container, you name it. And just a tip, compound means two words squished into one, ex. CupCake, FireWorks, BaseBall, BasketBall. But when you are typing it, of course, you wouldn't capitalize the first letter of the two words :)
It is usually the last sentence in the first paragraph and it tells what the essay will be about.
