Final answer:
The Soviets broke their b) commitment to allow free elections in Eastern Europe, as agreed at the Yalta Conference, which led to strains in postwar relations and contributed to the onset of the Cold War.
Step-by-step explanation:
At the Yalta Conference, the argument made by the Soviets that they later broke, causing tremendous tension in the post-war period, was b) the commitment to allow free elections in Eastern Europe. The Soviets, under Joseph Stalin, agreed at the Yalta Conference to allow free elections in European nations liberated from Nazi control, to create governments that reflected the will of the people. However, postwar, Stalin supported the establishment of Communist governments in Eastern Europe, backed by the Soviet military, contrary to the expectations set at Yalta for democratic processes.
These actions contributed to the origins of the Cold War, as they went against the allies' vision of a postwar Europe with democratic governments. The Western Allies had hoped to recreate Europe in their image with Western-style democracies but faced a Soviet Union intent on expanding its influence and promoting communism. As a result, a significant ideological divide grew between the Western powers and the Soviet Union, leading to a period of geopolitical tension known as the Cold War.