We already know that ~ 60/70% of the respondents will be new users, meaning that they will probably not understand properly what you're talking about. The WLM upload infrastructure has been built in such a way that we don't push the license too much in their face, but only inform them that they are releasing it etc - we don't push a choice to them. I don't expect any reliable answers from such a question (or any license related question). If the intended audience were primarily wikimedians, that might be different.
If any question around free licensing would be appropriate, it would probably be finding out whether it was a hurdle for them - do they consider releasing stuff under a free license a problem? But even about that I'm very doubtful whether it is worth the confusion and extra question.
Lodewijk