You have bought a Highway or a Bridge from the Black Market deck and then Farmland a Copper into a Silver is the only correct answer for your puzzle with that wording that I can think of, but you can also gain a Silver with Trader, Feodum, Catacombs, Haggler or Border Village as a result of buying a Farmland.
EDIT: Wait, there's another: You have a Quarry in play and Farmland a Steward into a Silver.