The correct option is A
The UN is governed by the Charter of the United Nations, which entered into force on October 24, 1945 and was signed on June 25 of the same year in the US city of San Francisco, by 51 countries, a few months before the end of the Second World War. The two world wars are explicitly mentioned in the preamble to the Charter.
The United States entered the United Nations, through a process of ratification that passed through the US Congress.