答案是pleasant..道理很简单,因为省略了for me / for you这样的结构：
This piano music is pleasant (FOR ME / FOR YOU ) to listen to.--------所以用主动语态listen to,不用被动语态结构to be listened to.
下面句子也是如此：
1 These books are interesting (FOR US) to read
2 This work is easy (FOR ME ) to do == This work is easily done (by me).
现在,你明白了吗?