According to the realist theory, why do international organisations such as the UN fail to prevent war?

Correct option is A

The correct answer is a) They lack enforcement capability.
In Realist theory, international organizations (IOs) like the United Nations are not seen as independent actors with their own power. Instead, Realists view them as "arenas" or "tools" used by powerful states to pursue their national interests.

Information Booster :

The Realist View: Why the UN "Fails"
Realism is built on the concept of Anarchy—the idea that there is no "world government" or higher authority above sovereign states. Within this framework, Realists argue that the UN fails to prevent war for several key reasons:

Reliance on Great Powers: The UN does not have its own standing army. It depends on member states (especially the P5: USA, Russia, China, UK, and France) to provide troops and funding. If a great power's interests are at stake, it will either ignore the UN or use its Veto power to block action.

The "Veto" as a Reality Check: Realists point to the UN Security Council veto as proof that the organization is designed to serve the interests of the powerful. If the UN tried to enforce a rule against a great power, that power would simply refuse to comply, rendering the UN's "enforcement" meaningless.

Self-Help System: Because there is no global police force with a "monopoly on the use of force," states must rely on self-help. They prioritize their own security and military balance over international laws or UN resolutions.

Epiphenomenal Nature: Realist scholars like John Mearsheimer argue that IOs are "epiphenomenal"—meaning they are merely a reflection of the underlying distribution of power in the world. They have no independent effect on state behavior; they only work when the interests of powerful states happen to align.