B.
The "things" that you can't remove from the sentence are the subjects. "Dropped" is the action, so that's the verb, which is required for a complete sentence.
Maybe you want to know who owned the sandwich, what kind of sandwich it was, what was in the box. Is any of that information required to understand that two things fell? No, they're describing the things that dropped onto the floor.
Is the backpack required? No, that's the place from which they fell. That's a nice detail, but it's not absolutely necessary.
Do you need to know that they fell onto the floor? No, it doesn't matter if they fell onto the floor, or a chair, or the sidewalk, or if they fell into some anti-gravity chamber and never landed anywhere. They fell, and that's all that's required to make the sentence.
Think of it this way:
You can rewrite this sentence to say, "the sandwich and box dropped onto the floor" and it would make sense. You could also say "the sandwich and the box fell," and that makes perfect sense. You need both items to tell the whole story, and you need an action [verb] to make it a sentence... otherwise you're just naming items. (It's true that you could remove one of the items and still make sense, but then you're telling half the story.)
