that---引导陈述句作宾语从句,无词意,可省略.
what和which都引导特殊疑问句做宾语从句.
what什么----没有一定范围
which那个（些）----有一定范围
What book do you like best?你最喜欢什么类书籍?
Which book do you like best of the three books?三本书中你最喜欢哪一本?