Historically, people in the united states have moved to cities for
a) employment and a strong cultural life.
b) a strong economy and better weather.
c) transportation and affordable homes.
d) affordable homes and better weather.

I believe the answer is: employment and a strong cultural life.

Most factories, warehouses, and offices tend to be built on cities in order to cut down distribution costs, which tend to lead to an increase in job opportunities. Strong cultural life also increase because many people with different ethnicity and cultural background tend to come and assimilate on the cities.