B
book是可数名词单数,前面必须有定冠词.所以this是修饰book的
那么这句话换成陈述句就是this book is （ ）you want to borrow from the library.
我觉的最好应该填the one ,但是没有,所以应该就填which,吧,指代this book,you want to borrow from the library是定语从句,省略了that.