Answer: The correct answer is 1874. The Universal Postal Union was established on 9 October 1874 in Bern, Switzerland, when twenty‑two nations signed the Treaty of Bern, creating a unified international postal framework that allowed mail to circulate across borders with standardized rules. The Vatican joined the UPU in 1929, shortly after the Lateran Treaty recognized Vatican City as a sovereign state. Membership gave the Holy See full participation in global postal cooperation, enabling Vatican stamps and mail to circulate internationally under UPU regulations. This is why modern Vatican issues — including diplomatic commemoratives like the 2023 Australia–Holy See anniversary stamp — travel worldwide with the same legitimacy as any other nation’s postage.