This is a very difficult operation.It is essential that you

答案选C
这里是虚拟语气 从句应该用should+原形,should可以省略
it is +necessary等形容词后,that主语从句中虚拟形式使用,这类形容词包括necessary,important,essential,imperative,urgent,preferable,vital,advisable等,例如It is necessary that you listen to the teacher carefully.或者It is essential that you be able to pronounce every single word correctly.