Explanation:

- The International Atomic Energy Agency (IAEA) is a global organization that encourages the peaceful use of nuclear energy and prevents its misuse.

- Option 1: London

London is a major global city but not the location of the IAEA headquarters.

- Option 2: Vienna

The IAEA headquarters is indeed based in Vienna, Austria, where it coordinates international nuclear efforts.

- Option 3: Vietnam

Vietnam is a Southeast Asian nation but not the home to the IAEA’s main office.

- Option 4: Washington D.C.

Washington D.C. is the capital of the United States but not where the IAEA is headquartered.

- Option 5: Rome

Rome is known for numerous historical and cultural influences, but it is not linked to the IAEA's main location.