"Welcome! Who are you?"
Translation:Fàilte! Cò thusa?
It does seem a bit odd to use thu with someone I don't know. Obviously it would depend on the social setting, and I might be addressing a child, but by default I would use sibh in this situation where I clearly don't know the person. Added to which there is nothing to say that only one person is being address. So I think, overall, that sibh is much more likely.
Having said this, it was offered to me as pick the right answer out of three. This one was possible and the others weren't, so it is actually a valid question, and it is quite right to set some questions with thu and some with sibh.