___that she will come.

B
it 作形式主语,that从句是真正的主语.
C答案不行,虽然语法正确,但是人们一般不这么说,再者,如果选择C的话,that引导的就是定语从句了,在此修饰possibility,即：There is some possibility that she will come,故错误.(⊙o⊙)
补充：你说C可以,且possibility也没有此用法啊!There is some possibility 要接的话一般接动词不定式短语或者介词短语,没有接定语从句啊!
哎,又遇到一个搞学术研究的了.faint-_-|||-_-|||-_-|||-_-|||