Answer: D. railroads
The key reason for Atlanta’s population growth during the 1800s is railroads.
Step-by-step explanation:
In 1836 Georgia decided to construct railway lines to the south west and Atlanta was chosen as the location for the terminus. Then the town grew as a settlement.
After some more years railway lines from four different directions arrived there and the town then grew to a rail hub for South US. Rail industry growth was crucial in the emerging of Atlanta as an important city.
Atlanta was also a significant location during the civil war because it was a major distribution hub. It was bombed by union troops but Atlanta recovered after civil war with the rise of manufacturing industries.