问题：定语从句

C.
先不看从句部分，主句不完整：“this book is”，所以这句话首先要使主句完整，也就是说，使定语从句有先行词。
主句：this book is the one （这本书就是那本）
从句：the one (that/which/省略）you want to borrow from the library.

如果题目改为“is this the book ___ you want to borrow from the library
则选A,B,D都行，因为还原主句“this is the book”已经是完整句，构成定语从句时指代“物”，又在从句做宾语，that,which,省略都可以

其实ABD都是可以的,如果先行词是做从句的宾语,可以用that,which引导,也可以省略

C- the one indicates specifically the book

因为只有用the one 才是完整的句子，还原成陈述句就是This book is the one you want to borrow from the library.其他选项均构不成完整的句子。

完整的形式是
Is this book the one which you want to borrow from the library?
which被省略了.
看一下句子的陈述语气,更好理解:
This book is the one which you want to borrow from the library.

主语: This book 谓语: is 表语; the one 定语从句: which you want to borrow from the library
如果没有the one, 句子就没有表语,就有语法错误. the one是定语从句的先行词,而which是从句的引导词,指代the one,因为which在从句中做宾语,所以可以省略.

C
这里the one 就是指的后面的you want to borrow from the library 的对象