it is necessary that she-once a week to help me with my less

答案选B would come
这里是虚拟语气,It is necessary that sb should do sth 走某事是很必要的.虚拟语气的结构是should / would /could do sth.
虚拟语气中的should不是可以省略吗?
不是所有的虚拟语气should都可以省略的,上面那种句型的就不可以省略. 当由suggest 引导的宾语从句的虚拟语气才可以省略
例如 I suggest that you (should) get up earlier.