You could click on that little box that says "no restrictions", and that would guarantee you a Y-class (and very expensive) fare.

nwa.com's reservation system will always give the lowest price it finds (or so they say... lower prices can be found via other methods). If you're looking for the lowest Elite-bonus qualifying fare, skip the 1000 mile bonus you'd get for booking on nwa.com (bigggggg deal) and call the Elite line directly and ask them how much a M-class ticket would be.