The main building of the United Nations is located in New York City. Optipn D is correct.
The United Nations has a headquarter in New York City, in a complex built by the architectural firm Harrison & Abramovitz. The complex constituted the official headquarters of the United Nations since it was completed in 1952. It is located in the Turtle Bay neighborhood of Manhattan.
The headquarters holds the seats of the principal organs of the UN, such as the General Assembly and the Security Council.