If you guessed C, you're right! A little water and elbow grease? Nada. Windex? Nah. Pinesol and steel wool. A little - a very little - success. The pad sander? Just a gunky pad, but semi-success.
The winner? Paint thinner and a rag to soften, and a flat blade to remove.
It took her about 3 hours to do the five stairs, but they're de-junked now. My project for the week is to wash them, and we'll sand and probably stain them next week. The edges aren't in as good of shape as the middle, we assume from extra adhesive. Yuck.