Wait why is kyā not allowed here? क्या वह लड़की कौन है?
Placing क्या at the beginning of a sentence turns it into a yes or no question, "क्या वह लड़की कौन है?" would translate to "Is this girl who?" which doesn't make much sense, unless the girl's name was literally "Who."
Ah, okay. That makes sense. Thanks!
Please demonstrate the difference between 'कौन' and 'क्या'.
कौन = who, क्या = what. "वह कौन है?" = Who is that/he/she? "वह क्या है?" = What is that?