对照一下这两句：
Is this book the one you bought yesterday?
Is this the book that you bought yesterday?
陈述句：
This book is the one (that/which) I bought yesterday.（the one是表语,后接定语从句）
This is the book (that/which) I bought yesterday.(有了表语the book,后接定语从句）
Is this museum the one (that/which) they visited last month?
陈述句：
This museum is the one（代表museum) （that/which) they visited last month.
不能这样说：
This is museum that they visited last month.
因为museum是可数名词单数,前面得加冠词或其它限定词.
This is the museum...才正确.
所以,中间填：the one (that/which)