The correct answer is: D. to consider a common plan of action by the Allies forces against the axis powers.
The Tehran Conference was a meeting of the leaders of the United States, the United Kingdom, and the Soviet Union, held in Tehran, Iran from November 28 to December 1, 1943. The three leaders were Franklin D. Roosevelt, Winston Churchill, and Joseph Stalin. The purpose of the conference was to discuss the strategy to be adopted by the Allies to defeat the Axis powers.
The conference was a success, and the Allies agreed on a number of important decisions, including the opening of a second front in Europe, the invasion of North Africa, and the bombing of Germany. The conference also helped to improve relations between the United States and the Soviet Union.
Option A is incorrect because the conference was not primarily about working out a common line of action against Japan. The Allies did discuss the situation in the Pacific, but this was not the main focus of the conference.
Option B is incorrect because the conference was not primarily about creating an effective instrument for maintaining international peace. The Allies did discuss the future of the United Nations, but this was not the main focus of the conference.
Option C is incorrect because the conference was not primarily about discussing the strategy to be adopted by the Allies to invade Germany. The Allies did discuss the invasion of Germany, but this was not the main focus of the conference.