The United Nations is primarily funded by its member nations. Member states contribute funds to support the regular budget of the United Nations, which covers essential expenses for the organization's operations. Assessments for the regular budget are based on a country's capacity to pay, taking into account its relative wealth and income. Additionally, the UN may receive voluntary contributions for specific programs or initiatives from member nations and other entities. Therefore, the correct answer is:

(D) Member Nations

The San Francisco Conference, also known as the United Nations Conference on International Organization, was held in 1945. During this conference, the final approval and adoption of the United Nations Charter occurred. Delegates from various countries gathered in San Francisco to establish the UN, and the charter, which serves as the foundational document of the organization, was signed on June 26, 1945. This marked a significant step in the post-World War II efforts to promote international cooperation, peace, and security.