Final answer:
Apart from delays, Carnegie faced serious setbacks in completing the bridge project due to labor strikes, which were common during that period due to the poor working conditions and low wages.
Step-by-step explanation:
Besides delays, another serious problem that plagued Carnegie as he tried to complete the bridge project was C) Labor strikes. The context given about Roosevelt's relief efforts and the challenges faced by workers indicates that labor issues were a significant concern during this period. Workers were often exposed to poor working conditions, low wages, long hours, and little protection—factors that commonly led to labor strikes. Such strikes would have directly impacted projects like Carnegie's bridge, causing serious setbacks.