What is the exact difference between “what” and “which” when distinguishing between alternatives? The best I can come up with is that “what” tends to be applied when there is a very large number of alternatives, but “which” is more suited to a small number of alternatives. For example, it is correct to ask a stranger “What is your name?” (it could be any of thousands), but if you and the stranger were looking at a short list of names, like a class roster, you might say “Which is your name?” meaning more explicitly, “Which of these names is your name?”
I’ve noticed a tendency for people to use “what” when they used to say “which”. For example, “Your position on welfare tends to correlate with which political party you prefer” has somehow morphed into “Your position on welfare tends to correlate with what political party you prefer.” It just sounds a little wrong.
“Which” is correct for identification of a selection among alternatives but colloquially people say “what,” it’s very common, nobody worries about it too much, and by that token it might even correct. (I studied some Italian a few years ago and recall one text that made exactly the same point in Italian regarding *che *and cosa.)
“What” is correct here, but not because there are so many names to choose from. It is correct because you are not asking for a choice, you are asking for an arbitrary piece of information. If you were standing in front of the Vietnam Memorial with something like 60,000 names, standing next to a widow, it would be correct to say, “Which name is your husband’s?” The magnitude of the number doesn’t matter.