The correct answer is: A. 24 October
UN Day is celebrated on 24 October each year. It is the day on which the United Nations was founded in 1945. The UN is an international organization that works to promote peace and security, develop friendly relations among nations, and achieve international co-operation in solving international problems of an economic, social, cultural, or humanitarian character, and in promoting and encouraging respect for human rights and for fundamental freedoms for all without distinction as to race, sex, language, or religion.
Option B is incorrect because 24 September is the day on which the UN General Assembly was established in 1945.
Option C is incorrect because 26 June is the day on which the UN Charter was signed in 1945.
Option D is incorrect because there is no option that is none of the above.