Final answer:
The drop in stock prices in late 1929 was primarily caused by the Stock Market Crash.
Step-by-step explanation:
The drop in stock prices in late 1929 was primarily caused by the Stock Market Crash. This crash was the result of several factors, including the speculative bubble in the stock market, excessive use of margin to buy stocks, and insider trading. The crash led to a chain reaction of bank failures and a decline in consumer demand, which contributed to the Great Depression.