Which of these is most likely to lead directly to a black market?

a. a supply shock

c. rationing

b. a price floor

d. equilibrium

I would say it's rationing: if people are only allowed a certain number of something (let's say: bottles of cooking oil), then the people who are entitled to is (as all are) but don't need is as much are likely to try to make profit by trying to sell it on a black market.