Which of the following is the most significant cause of the economic struggles of the new south?
a. the democrats gained political control, limiting the development of new industry.
b. southern farmers had not learned to manage cash crops.
c. the south had been too dependent upon the plantation system.
d. the southern states would not trade with other states.

The best answer is C.

The south depended too much on the plantation system and slave economy which was labor intensive as opposed to the north which was rapidly becoming mechanized , with production of  agricultural products and manufacture of goods  creating a major economic stride forward.

Though they produced two thirds of the world's cotton, the south had little manufacturing capability. They only had about 29% of the railroad tracks and only 13% of the nation's banks.

Free states attracted the vast majority of the waves of European immigrants.  The North had 23 million people as compared to 9 million in the south which negatively impacted the southern economy but positively impacted the northern economy.