The united nations was established so that it could resolve conflicts between nations by a. launching military attacks. b. forming blockades. c. imposing sanctions. d. occupying border countries

The correct answer is C: 
imposing sanctions.

Answer (from these choices):  C. imposing sanctions

Explanation / detail:

The United Nations describes its own purpose and plan for resolving conflicts as one of promoting international cooperation and respect for the principles of justice and international law.  Sanctions may play into that as a means of enforcement of UN principles.

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.