在你的句子里面,
1 如果是修饰定语从句部分,which就是正确的答案,但句子不完整.完整的句子应该是（比如说）：It was this book___which__ I had been looking for then that Tom had taken away without telling me.--------- 非强调句的语序是：Without telling me,Tom had taken away this book which / that I had been looking for.
2 如果是强调句的话,缺少连接词that,所以,你给的选项都是错的.
It was this book___that__ had been looking for.