A prodrug is a compound that is not active on its own but becomes active only after the body converts it into the true active drug, and the story of sulfonamide antibacterials actually begins with exactly this kind of compound.
Gerhard Domagk tested Prontosil, a red azo dye, and found it protected mice from a lethal streptococcal infection, yet when the same dye was tested directly against bacteria growing in a dish, it showed no antibacterial effect whatsoever. That contradiction was solved when researchers discovered that Prontosil is broken down inside the body, the azo bond is cleaved, releasing sulfanilamide as the actual antibacterial agent. Prontosil itself never touches the bacteria directly, it works purely through this in-vivo conversion, which is the exact definition of a prodrug.
Sulfatrim and trimethoprim both belong to a much later period of sulfonamide and antifolate drug development and are not connected to this original discovery, and trimethoprim is not even a sulfonamide chemically.
Because Prontosil is the compound historically and consistently credited with this discovery, it is the correct answer.