Which industry benefited the most from the construction of railroads in the post civil war period?
a. agriculture
b. banking
c. steel
d. mining

C, the Steel industry, as railroads were constructed from steel beams.

I feel like that is almost definitely Agriculture. For one, it just makes logical sense because railroads = industrialization and industrialization = more people (aka more crop output). But if you need to examine it more, it's obviously Agriculture because now farmers could sell their crop at a national level and not be dependent on the market of their local town.