Which UN organ has the authority to discuss issues concerning international peace, security, and the admission of new members?

The Security Council is the UN organ specifically tasked with maintaining international peace and security. It has the authority to discuss and make decisions on issues related to conflicts, threats to peace, and aggression. This includes the imposition of sanctions, authorization of military action, and the establishment of peacekeeping operations.

While the General Assembly also plays a role in discussions about peace and security, particularly through resolutions and recommendations, it does not possess the binding authority that the Security Council does. The Economic and Social Council primarily focuses on social and economic issues, and the International Court of Justice handles legal disputes between states and gives advisory opinions on legal questions, but it does not discuss issues of international peace directly. Therefore, the Security Council's unique mandate and powers make it the appropriate body for such serious matters as the admission of new members and discussions on international peace and security.