首先,如果用what的话,what is required 是一个名词性从句（即,它的作用相当于一个名词）,在句子中做主语,完整句子是这样的：What is required in the regulations “is” that you should not tell other people the password of your email account.注意到了吗?你需要加一个is在that前面.这句话等于：The requirement is that...
如果that前面有is,这道题就要选A了.可现在that前没有is,那么这句话就应该这样分析：
It is required that you should not tell other people the password of your email account.意思是什么?that后面的句子才是要求的具体内容,如果正常的语序应该这么说： "You should not tell other people the password of your email account" is required.引号里面是才是要求的具体内容,才是真正的主语,可这样说是不是给人非常头重脚轻的感觉?于是,it显神通的时候到了.用it代替整个主语放到前面,用that来把主语引导出来,这样就解决了整个问题.
楼主你懂了吗?