The longest river in the world is the Nile. Therefore, the correct answer is (D) Neil (Nile). The Nile River is a major north-flowing river in northeastern Africa and is generally considered the longest river globally, with a length of approximately 6,650 kilometers (4,130 miles).

The headquarters of the International Atomic Energy Agency (IAEA) is located in (B) Vienna, Austria. Vienna has been the headquarters of the IAEA since its establishment in 1957. The city's selection as the headquarters reflects its role as a neutral and international city, making it a suitable location for an agency tasked with promoting the peaceful use of nuclear energy and preventing its use for military purposes.