The International Atomic Energy Agency (IAEA)

1. controls nuclear weapons stockpiles.
2. inspects nuclear power plants.
3. tests nuclear weapons.
4. builds nuclear power stations.

I think it's number 4

http://www.google.com/search?q=IAEA&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-US:official&client=firefox-a

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.

copied from wiki

4. builds nuclear power stations.
seems to be the only acceptable answer

Actually, the correct answer is number 2: "inspects nuclear power plants." The International Atomic Energy Agency (IAEA) is an international organization that promotes the peaceful use of nuclear energy and helps prevent the proliferation of nuclear weapons. While it does not control nuclear weapons stockpiles or test nuclear weapons, it does have authority to inspect nuclear facilities, including power plants, to ensure they are being used for peaceful purposes and in compliance with international safeguards. The IAEA plays a crucial role in monitoring and verifying nuclear activities worldwide to ensure the safety and security of nuclear facilities and prevent the spread of nuclear weapons technology.