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.

is there anyone who can help?

Study this:
A:Who was that at the door?
B: I have no idea who that was at the door.

Study this:
A:Who was that at the door?
B: I have no idea who that was at the door.
I agree with you very much for your wonderful explanation

can't we say " who was at the door?
what is the meaninig of
' that
' here?
could you explain a littel more please