Is this book_____ you bought the other day?A.that Bwhich Cthe one 但是为什么不选B 可不可以把它还原成陈述句之后 理解成一个表语从句呢?this book is which you bought the other day .把which you bough理解成表语.

如果作表语应该是this book is what you bought the other day .同时 可改写为this book is the one which/that you bought the other day .因为the one 在定语从句中作宾语，which/that 可省略,为this book is the one you bought the other day ,

which 不能单独用来指代前面的the book ，其实C选项就像楼上说的，是相当于： the one which, 其中which可以省略，但是你总不能省了 the one, 这样先行词都没了，怎么构成从句。
另外你理解成表语从句，which 应该改为what， bought which 是不合语法的。

which you bought 的正确用法是
Is this book (the one which) you bought the other day?
which 通常可以省略，所以
正确答案是 C. the one

当你用which后面的句子就变成附加句，整句话的意思变了。
Is this book which you bought the other day? = Is this book? which you bought the other day.

首先,这是一个定语从句,the one 后面其实省略了一个that,that在定语从句中,作为从句中的bought的宾语,可以省略.再者,the one 是特指the other day 这一天借的书,不能省.最后,This is which you bought the other day,是不正确,应该是 This is what you bought the other day.因为which 不仅仅坐表语,还作为 bought 的宾语.