The correct option is 1, October 24, 1945. The United Nations (UN) is an intergovernmental organization that was established to promote international peace, cooperation, and security. It was founded after World War II to replace the League of Nations, which had failed to prevent the outbreak of the war. The UN was officially established on October 24, 1945, when the UN Charter was ratified by the majority of its original 51 member states.

Option 2, October 24, 1943, is incorrect. This date is not accurate as the UN was not in existence at that time.

Option 3, November 26, 1945, is also incorrect. The UN was already established by this date, so it cannot be the date of its establishment.

Option 4, November 26, 1943, is incorrect for the same reason as option 3. The UN did not exist on this date.

Therefore, the correct answer is option 1, October 24, 1945, as this is the date when the UN officially came into existence.