正确的答案是 B.where
The library (where) you often borrow books is under repair now.
虽然我们说 borrow books from the library
但以 library 开头的句子习惯说法是
the library where you borrow books
原因是听者知道 books 后面有一个省略的 from .
其他类似句子：
The bank where you borrow money ...
The store where you bought the shirt
等等.
我是加拿大人,英语母语.