Final answer:
The Republic of San Marino holds the title of the oldest surviving sovereign state and constitutional republic. The Constitutions of Melfi, while notably old, relate to the Kingdom of the Two Sicilies and not San Marino. The U.S. Constitution is recognized as the oldest operational written national constitution.
Step-by-step explanation:
The Oldest Surviving Sovereign State and Constitutional Republic
The oldest surviving sovereign state and constitutional republic in the world is the Republic of San Marino. It claims to be the oldest constitutional republic, having been founded on 3 September 301 by Marinus of Rab, and it has maintained continuity since then. Although the Constitutions of Melfi are recognized as the oldest written constitution in the world, which brought significant changes to the Kingdom of the Two Sicilies by increasing the monarch’s power, it is not affiliated with the world's oldest surviving state.
In contrast to the Constitutions of Melfi, the United States Constitution, ratified in 1788, is recognized as the oldest written and codified national constitution in operation. Despite being centuries younger than the founding of San Marino, the U.S. Constitution has acted as a template and influence for other modern national constitutions globally.