Which one of the following international institutions plays an important role in determining economic policies across the world?

The correct option/answer is IMF.

To understand why this is the correct choice, let us examine the roles of each of the international institutions listed in the options:
1. IMF (International Monetary Fund): The IMF is an organization of 190 countries working to foster global monetary cooperation, secure financial stability, facilitate international trade, promote high employment and sustainable economic growth, and reduce poverty around the world. It plays a critical role in determining and shaping economic policies globally, particularly by providing policy advice and financing to members in economic difficulties.
2. IAEA (International Atomic Energy Agency): This organization seeks to promote the peaceful use of nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It has no role in global economic policymaking.
3. Amnesty International: This is a non-governmental organization focused on human rights, with over 10 million members and supporters around the world. It does not determine economic policies.
4. Human Rights Watch: This is an international non-governmental organization that conducts research and advocacy on human rights. Like Amnesty International, its focus is on human rights rather than global economic policies.

Therefore, among the given options, the IMF is the international institution that plays a direct and significant role in determining and influencing economic policies across the world.