A drop of ink disperses through a glass of water and never spontaneously regathers. Yet each molecular collision is perfectly reversible. What actually forbids the ink from re-forming?

C. Nothing forbids it; it is overwhelmingly improbable

Every microscopic motion could legally reverse; there are simply astronomically more ways to be mixed than gathered, so chance never finds the gathered ones again. Entropy increase is probability at work, with $10^{23}$ molecules, "overwhelmingly likely" is indistinguishable from "always".