ارجو الافادة من السادة الزملاء

[الاسوانى محمد حسين]

dear colleaues;
in longman ex unit 13 grammar. sentence no(9)

I have no idea who _____ was at the door.

a) who b) which c) where d) that
it was written that the right answer is (that) please , would you be patient and tell why ???
thank you for your time.