The correct answer is option 3, the San Francisco Conference. The United Nations Charter was signed by 51 original members of the United Nations at this conference, which took place in San Francisco, California. It occurred from April 25 to June 26, 1945. During this conference, representatives from different countries gathered to discuss and draft the charter for the newly formed international organization, the United Nations. The purpose of the charter was to establish the goals, principles, and structure of the United Nations, as well as outline its duties and responsibilities. The signing of the charter marked a significant moment in the establishment of the United Nations, as it demonstrated the commitment of these 51 nations to work together to promote peace, security, and cooperation on a global scale.