After the Second World War, the charter of the newly formed United Nations established an eleven- member Security Council and charged it with taking collective action in response to threats to world peace.
Conclusion
The five nations that were major powers at the time of the UN's founding should permanently have sole authority to cast vetoes.
Evidence
The maintenance of world peace would rest on the world's major powers, and no nation should be required to enforce a decision it found repugnant.
Evaluate
There's a missing link between the Problem identified (a world power might have the burden of enforcing a UN decision it found repugnant) and the Solution (give them veto power).
That might sound like,
We might also get more practical and engage our Plan / Goal faculties. The Goal here was to avoid a world power having to enforce a decision it disagreed with, and the Plan is to provide only those five countries with veto power.
How could this plan fail to achieve its goal?
Could a major power somehow be pressured to not use its veto power, restrained from using its veto power? Could it veto something but still have the burden of enforcing the decision it vetoed?
Goal
Hard to have a great guess of where this is going. Maybe it'll be a missing link that connects the Problem and the Solution.
Maybe it'll be a Defender answer, where if we negated it, it would sound like the Plan won't achieve its Goal.