Using diplomatic as a way to end conflict between nations is one of the function of

the un
ngos
the chief negotiator
the un general assembly

Correct

The UN(The United Nations)

Details:

The United Nations (UN) seeks to settle international differences and disputes by promoting diplomacy, international cooperation and respect for the principles of justice and international law.  The UN Charter, signed in 1945, details the purposes of the organization in Chapter I, Article 1.  Chapter II, Article 2 speaks of how those goals will be pursued.  Here's some of the official wording:

From Chapter I, Article 1  [describing the first purpose of the UN]:  To maintain international peace and security, and to that end: to take effective collective measures for the prevention and removal of threats to the peace, and for the suppression of acts of aggression or other breaches of the peace, and to bring about by peaceful means, and in conformity with the principles of justice and international law, adjustment or settlement of international disputes or situations which might lead to a breach of the peace;From Chapter I, Article 2  [describing how it pursues conflict resolution]: All Members shall settle their international disputes by peaceful means in such a manner that international peace and security, and justice, are not endangered.     All Members shall refrain in their international relations from the threat or use of force against the territorial integrity or political independence of any state, or in any other manner inconsistent with the Purposes of the United Nations.