"Ποιος" = Which; "τι" = What?
It's not a one-to-one mapping with English though. There are many examples where English uses "what" but Greek needs ποιός and not τι.
"Which one is your account" sounds better than "What is your account?"
A strange question. I can understand being asked for my account number, or my username, or even my name or my account balance. Maybe there's a better translation?
I was thinking the same.
Maybe a more realistic sentence example would be "Τι όνομα χρήστη έχεις στο...(insert app name);" (What is your user name in...? [lit: What name of user you have in...?] )
I thought the question was "which is your bill?" and was surprised to be told it was wrong
It's ok now.