The correct answer is C. permanent members of the Security Council.
The P-5 are the five permanent members of the United Nations Security Council: China, France, Russia, the United Kingdom, and the United States. These countries were the five major Allied powers in World War II, and they have veto power over any Security Council resolution.
A. highly developed countries is not the correct answer because there are many highly developed countries that are not permanent members of the Security Council, such as Germany, Japan, and South Korea.
B. highly populous countries is not the correct answer because there are many highly populous countries that are not permanent members of the Security Council, such as India, Indonesia, and Brazil.
D. established nuclear powers is not the correct answer because there are many established nuclear powers that are not permanent members of the Security Council, such as India, Pakistan, and North Korea.