Among the given choices, the sentence fragment is b. Books, papers, and folders in a stack on the desk.
Sentence fragment are called as such because fragments are found in incomplete sentences. Fragments cannot stand on its own. It does not convey a complete thought. Fragments must be connected to an independent clause to form a complete sentence and convey a complete thought.
In the sentence fragment <em>books, papers, and folders in a stack on the desk</em> <em>, </em>it does not give a complete message because it gives rise to the question What? What about the books, papers, etc?
The story says, "Then, gently put the valve of the tube through the hole in the rim and insert the rest of the tube into the tire, being careful not to twist it."
