Inversion (who is the man standing at the door.)

[Sitifan]

I don't know _____________________.
(A) who is the man standing at the door
(B) who the man standing at the door is
The answer is B. Is choice A really not acceptable?
_________________
Thank you very much for your reply.

[Yankee]

Hi Sitifan

Option (B) is grammatically correct, and (B) is also the option that sounds natural to me.
.

[Molly]

"I don't know" suggests a question to be answered, there.

It works in similar way to:

Could you tell me who the man standing at the door is?
It's a mystery who the man standing at the door is.
May I ask who the man standing at the door is?

etc.

[Haihao]

Sorry for the digression but I have lost my memory about the difference in locality between 'at the door' and 'by the door'. Could you please shed some light on it?

[Yankee]

Hi Haihao

If you say 'at the door', I picture someone standing in front of it, probably facing it, perhaps knocking or ringing a doorbell to be let in.

If you say 'by the door', I picture something next to or in the vicinity of the door. For example, you might keep your umbrella or a pair of boots by the door.
.

[Molly]

[Yankee]

[Lost_Soul]

Actually, in my country bad boys are made to stand in the corner.

[Yankee]

Hi Alex

'Stand in the corner' does sound more likely, doesn't it.
.

[Molly]

[Molly]

Which one of these would "the man standing at the door" belong to?

[Haihao]

Hi Amy,

Thank you so very much again and I'd like to add that your pictures both answered my questions and cleared my doubts away completely.

Haihao