Answer: B
Correct answer: Books
The question asks what is always present, not what usually is. A library is by definition a collection of books or other written material, so books are what make it one - nothing without them is a library. The others are common but not necessary. Many libraries have computers, but a village reading room with a shelf of books is still a library. A separate reading room is a convenience, and small collections have none. And a librarian is a person employed to run a library, not part of the definition - a private library kept by its owner has no librarian at all.