In this case I think what should be accepted. So it's a missing alternative translation.
However take care, in general regarding the mouse-over tooltips: not all possible translations work in all contexts.
FYI both what and which can be translated to welk. In Dutch the distinction between a limited or unlimited set of options doesn't exist.
I reported this 5/5/15. I think "what" is a stronger answer in this situation because "which" indicates that you have to choose your answer from a definite set of answers. Without context, there are limitless ways to wage a war.