Atropine blocks acetylcholine receptors on the heart. Given to a resting patient it raises the heart rate substantially, although it does nothing whatever to the sympathetic system. Why does blocking a receptor speed the heart up?

C. Because the resting heart is under constant drive from both branches.

The word doing the work is 'resting'. A resting heart is not idling free of input, it is being held down by continuous vagal brake, and atropine takes the foot off it. Blocking a receptor removes a signal rather than generating its opposite, the speeding was there all along, unopposed.