Following the war of 1812, the construction of us railroads and growth of us cities showed that
the economy was growing quickly.
the united states had won the war.
great britain had lost the war.
the economy only grew in cities.

The Economy was growing quickly

Explanation:

Despite the war, American economy grew quickly because the cities of the US started to grow rapidly. As the cities were growing, overall economy of the US was growing. It was easy for the US  to recover from the effects of the war. That's why option A or The Economy was growing quickly is correct.

your answer would be (b)