c. increased production of goods.
The main reason why the standard of living improved in the United States after World War II was the increased production of goods. During the war, American factories had been converted to produce military equipment and supplies, but after the war, they were able to quickly shift back to producing consumer goods. This led to a period of economic growth and prosperity in the United States, as Americans were able to purchase a wide range of goods and services that had not been available during the war.
The other options, demobilization of the military, postwar devastation overseas, and development of atomic energy, were all important factors during the post-World War II period, but they were not the main reason why the standard of living improved in the United States. Demobilization of the military led to a reduction in government spending, which helped to stabilize the economy, but it did not directly contribute to increased production of goods. Postwar devastation overseas created opportunities for American businesses to expand into new markets, but this was a secondary factor in the overall improvement in the standard of living. The development of atomic energy had important implications for science and technology, but it did not have a significant impact on the economy or the standard of living in the short term.