Of the following laws, which one required that the federal legend appear on all prescriptions?

The law that required the federal legend to appear on all prescriptions is the Durham-Humphrey Act of 1951.
The Durham-Humphrey Act, also known as the Prescription Drug Amendment, amended the Federal Food, Drug, and Cosmetic Act of 1938. It established a distinction between prescription drugs and over-the-counter (OTC) drugs and defined the criteria for drugs that require a prescription. It introduced the "Caution: Federal law prohibits dispensing without a prescription" statement, commonly known as the federal legend, which must appear on prescriptions for drugs that are restricted to prescription use only.