Middle-class suburbs of the late nineteenth century were characterized by: a) their isolation from cities.
b) high property taxes.
c) urban sprawl.
d) high population densities.

Option: c) urban sprawl.  

Explanation:

Middle- class suburbs in the late 19th century characterized by the uncontrolled expansion of urban areas. The industrialization resulted in overcrowding in cities and towns. Majority of the rural people moved out of their county side and moved to urban to find a better job opportunity and to start a new life. The moving of the rural population in the urban areas led to the rise of urbanization.

i believe your answer is mountains.