"Who is in what position" is the way it would normally be said in spoken English, even though technically it seems like "which" would be correct. You have raised a good question because in some languages, there is a big distinction between "what" and "which" (like in French, for example). However, in English, using "what" in this way is perfectly natural. We tend to use them rather interchangeably.

Unkraut, Both are possible, but they have different implications. For example, if I am a teacher and I do not have a list of the students that should attend my class, if one of the students comes and I do not know his name, I can say "what is your name?". But on the other hand if I have a list of the students that should attend the class, when the student comes in I can point to the list and say "which is your name?". Basically the difference is whether you know the options or not. If the options are infinite or just unknown, you use "what". What car do you drive? (infinite number of options) But if we are in front of 20 cars parked, I can ask "which is your car?" because the options are in front of me. I can ask "what is your favourite CD? (infinite number of options) But if you have a few CD's on your desk, I can ask "which is your favourite CD?" because the options are known. I hope this helps.