The correct answer is A) the ability to declare war on behalf of the United Nations.
The unique power that the members of the United Nation's Security Council hold is the ability to declare war on behalf of the United Nations.
According to the official website of the United Nations, the Security Council maintains peace in the world in accordance with the principles of the United Nations. It also tries to resolve any dispute among countries to try to prevent any war. The Security Council can take military action if it considers necessary to repel an aggressor. So it is true that the unique power that the members of the United Nation's Security Council hold is the ability to declare war on behalf of the United Nations.