What to do with an application question when I honestly don't know my plans?
Correct me if this doesn't apply to your case but applications generally provide one or several areas where one can explain their interests and future plans (e.g. statement of purpose). Thus, I doubt that this question is all that important. It might even just be used for administrative purposes within or outside the department.

My take on this is that - so long as you are completely honest about your intentions/uncertainty elsewhere in the application - fudging/randomly choosing your response isn't that big a deal. 

So what option should you circle? I would go back and reread the program/department webpages to get an idea of graduate placement. (It is also completely legitimate to ask for past placement if it isn't posted.) If most people go on to grad school, circle yes. If most don't, no. So long as you don't feel like you're lying as you circle, nobody can fault you. They should have provided a "don't know" option in the first place. 

Finally, I'm not a big fan of the "NA" response because (a) the question is, in a sense, applicable to you and (b) it may raise unnecessary red flags.