At the Yalta Conference, the ‘Big Three’ decided to organize a United Nations Conference on the proposed world organization. The ‘Big Three’ stood for:

The correct option is Roosevelt, Churchill, and Stalin.

Step-by-step Explanation:

1. Context of the Yalta Conference: The Yalta Conference, also known as the Crimea Conference, was held from February 4 to 11, 1945, during World War II. Its purpose was to discuss the postwar reorganization of Germany and Europe, as well as the establishment of a new global organization to maintain peace.

2. Identifying the 'Big Three': The term 'Big Three' refers to the heads of government of the three major Allied powers of World War II: the United States, the United Kingdom, and the Soviet Union. Specifically, these leaders were:
• Roosevelt: Franklin D. Roosevelt, President of the United States.
• Churchill: Winston Churchill, Prime Minister of the United Kingdom.
• Stalin: Joseph Stalin, Premier of the Soviet Union.

3. Eliminating Incorrect Options:
• Khrushchev was not in power during World War II; he became the leader of the Soviet Union much later, in 1953.
• Deng Xiaoping was a leader of China, and he was not one of the Allied heads of state at Yalta.
• John F. Kennedy became the U.S. President in 1961, and Clement Attlee replaced Winston Churchill as Prime Minister of the United Kingdom only in July 1945, after the Yalta Conference had already taken place.

Therefore, the 'Big Three' at the Yalta Conference represented Roosevelt, Churchill, and Stalin.