选D
完整的句子应该是：
“What does Betty think of the book which was written about Harry Potter?”（带定语从句）
可以简化为“What does Betty think of the book written about Harry Potter?”（过去分词做后置定语）
变成第二个形式时,be动词也要省略.
因为没有which或者that,不是定语从句,排除AB
C是过去时,不是过去分词,排除.
【公益慈善翻译团】真诚为您解答!