C, under consumption
overproduction was a major issue as farmers industrialized
Stock speculation was a major issue, and the main cause of the great depression - People would invest in stocks without doing proper research.
Foreign competition, as a real 'global' economy developed, was a issue, and the world became extremely inter-linked as it is now; a economic crisis in one nation will affect countless others.
Under consumption was not really a contributing factor as people had more money to consume more items leading up to the great depression (1920's)