(mc)which resulted from the growth of the middle-class in the united states during the 1950s? low birth rate expansion of suburbs increase in immigration decline in defense spending

Expansion of the suburbs.

As far as the option about "low birth rate," that's very much wrong.  The period from 1946 to 1964 is known as the "baby boom" period in the United States.  More babies were born in America than ever before in 1946, and the high birth rates continued for a number of years.  Birth rates in the late 1940s and 1950s were much higher than they had been before, especially during the very low years of the Great Depression and World War II.

For more thought on the growth of the suburbs in the 1950s, read more on -

Expansion of suburbs is the correct answer, I just took the test.