The correct answer is D) railroad tracks and total population.
During the Civil War, the resource areas where the Union held 71% of total resources were railroad tracks and total population.
As we can see in the graphic attached, the Union considerably had more resources than the Confederacy. For instance, regarding horses and food crops, the Union had 72% for each one. The Confederacy lied way behind with just 28%. Regarding soldiers, the Union had 67% and the Confederacy 33%. In railroad tracks, the Union had 71% and the Confederacy 29%. In factories, the Union had 86% and the Confederacy just 14%. Regarding bank deposits, the Union had 81% and the Confederacy 19%. Finally, regarding population: the Union 71%, the Confederacy, 19%.