The correct option is A Vienna, Austria
The International Atomic Energy Agency (IAEA) is an international organization that seeks to promote the peaceful use of nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was established as an autonomous body on 29 July 1957, with it's headquartered in Vienna, Austria. It has a current membership of 171 nations. The organization also has two "Regional Safeguards Offices" which are situated in Toronto, Canada, and in Tokyo, Japan.